fix(EN-1441): propagate initIndexConfig boot errors with self-healing retry

[paul-nicolas]

Summary

Builder.initIndexConfig returned void and logged-and-returned on every boot error branch, so Builder.loop could not observe the failure and unconditionally advanced to processLogs. A transient Pebble/read-store failure at boot left the in-memory index config empty or partially populated — the write path then silently skipped index writes (byCanonical miss ⇒ isMetadataIndexed/isBuiltinIndexed return false) while the persisted cursor kept advancing, leaving the read-side index permanently incomplete with no error, no retry, and no crash.

Fix

• initIndexConfig(ctx) error (was void): the four boot read branches now return a wrapped error instead of a swallowed log. A top-of-function reset of indexConfig/bucketIndexConfig/backfillTasks/schemaRewriteTasks/indexVersions makes it idempotent across retries (the two slices would otherwise double-schedule).
• Extracted the boot prologue into bootInit(ctx) (cursor, pebbleLast, err) and wrapped it in the existing worker.RetryWithBackoff (100 ms → 10 s, honors the stop channel). A ctx.Err() check after the retry distinguishes "init succeeded" from "shutdown requested" so loop never processes logs against a failed init.
• This also closes the identical silent-return defects at the sibling LastIndexedSequence and NewDirectReadHandle boot reads. ReadAppliedProposalProgress / ReadLastSequence keep their existing best-effort semantics.

A transient boot read failure now retries with backoff instead of silently advancing the cursor against an incomplete config.

Tests

• TestInitIndexConfig_PropagatesReadError — a closed store makes the boot read fail; the error propagates instead of being swallowed.
• TestInitIndexConfig_IdempotentAcrossRetries — calling initIndexConfig twice does not double-schedule backfill/rewrite tasks.
• TestBootInit_PropagatesError / TestBootInit_Success — cover the extracted prologue.
• worker.RetryWithBackoff mechanics are already covered by internal/pkg/worker/worker_test.go.

go build ./..., just pre-commit (0 lint issues), and go test ./internal/application/indexbuilder/... all pass.

Ticket

https://formance-team.atlassian.net/browse/EN-1441

Same silent-swallow class as EN-1440.

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[codecov]

Codecov Report

❌ Patch coverage is 72.97297% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.28%. Comparing base (5ddbe29) to head (8c355cb).
⚠️ Report is 1 commits behind head on release/v3.0.

Files with missing lines	Patch %	Lines
internal/application/indexbuilder/builder.go	80.76%	2 Missing and 3 partials ⚠️
internal/application/indexbuilder/index_config.go	54.54%	3 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (72.97%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v3.0    #1482      +/-   ##
================================================
- Coverage         73.22%   71.28%   -1.94%     
================================================
  Files               388      388              
  Lines             39765    40022     +257     
================================================
- Hits              29117    28531     -586     
- Misses             7900     8106     +206     
- Partials           2748     3385     +637     

Flag	Coverage Δ
e2e	71.28% <72.97%> (-1.94%)	⬇️
scenario	71.28% <72.97%> (-1.94%)	⬇️
unit	71.28% <72.97%> (-1.94%)	⬇️

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