encoding/json: fix Indent with a non-space prefix or indent

[lazerg]

When the prefix or indent contains characters that v2 rejects,
the v1 emulation formats with placeholder spaces of the same length
and rewrites them with the original strings afterwards.

The rewrite ran in a defer, so it ran after the trailing whitespace of
src had already been appended and overwrote that whitespace as if it
were indentation; with an empty indent it looped forever.
Move the logic before the trailing whitespace is appended
and avoid a defer.
Also, add a test to both v1 and v1in2 to codify this behavior.

This only modifies code that is compiled in under goexperiment.jsonv2.

Fixes #81547

[gopherbot]

This PR (HEAD: 2664e2b) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/833524.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

This PR (HEAD: b3d480e) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/833524.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Emmanuel Odeke:

Patch Set 2: Commit-Queue+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-09-16T07:19:31Z","revision":"d5433df617ab3e03b861c80d2bba645573693f86"}

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Emmanuel Odeke:

Patch Set 2: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_5137>)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

This CL has passed the run

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2: LUCI-TryBot-Result+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Joseph Tsai:

Patch Set 2: Code-Review+2 Commit-Queue+1

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-10-09T18:12:25Z","revision":"d5433df617ab3e03b861c80d2bba645573693f86"}

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Joseph Tsai:

Patch Set 2: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_8495>)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

This CL has failed the run. Reason:

Tryjob golang/try/x_tools-gotip-linux-amd64 has failed with summary (view all results):

---

• go on master (change 833524) / tools on master (commit 1e27b6e)

To reproduce, try gomote repro 8668396880889552913.

Additional links for debugging:

• test golang.org/x/tools/gopls module output

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2: LUCI-TryBot-Result-1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/833524.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: ef1869a) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/833524.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.