refactor: share file upload operation

[victor0602]

What changed

• Extract the file upload path validation, multipart construction, endpoint selection, and request execution into one internal operation.
• Reuse that operation from both the CLI command and FileSDK.
• Keep CLI prompting, dry-run, quiet output, and SDK-specific errors unchanged.

Why

The CLI and SDK previously maintained separate upload implementations, so fixes could land in one path and be missed in the other.

Impact

CLI and SDK behavior stays compatible while the upload protocol now has a single implementation and stronger multipart coverage.

Checks

• bun test — 450 passed
• bun run typecheck
• bun run build
• bun run lint — no errors; one pre-existing test warning
• git diff --check

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[NianJiuZst]

No blocking findings on the current head. The shared upload operation preserves CLI dry-run/output behavior and SDK-specific errors while consolidating multipart construction, so I recommend merging it.

Verified locally: typecheck, lint (one pre-existing warning), build, 7 focused upload/SDK tests, and the full suite (450/450). If #228 lands first, rebase and re-run the file SDK tests because both PRs touch src/sdk/file/index.ts.

[NianJiuZst]

No blocking findings after revalidating this head against the latest main. The shared upload operation preserves CLI dry-run/output behavior and SDK-specific error identities while consolidating multipart construction, so I recommend merging it after #208.

Verified: focused multipart tests, typecheck, lint (one pre-existing warning), and the combined #208 + #229 full suite (402/402).