`AllExceptionsFilter` test for every documented error envelope

[kilodesodiq-arch]

Problem Statement. error-handling.e2e-spec.ts covers some shapes; it's unclear
that every code path is documented.

Technical Context.
app/backend/src/common/filters/http-exception.filter.ts,
app/backend/test/error-handling.e2e-spec.ts.

Expected Outcome. A regex-based test enumerates every @ApiResponse({ status: 4xx }) or 5xx }) in the Swagger output and asserts there is a corresponding test.

Acceptance Criteria.

• CI runs the matrix; missing tests fail loudly.

Files or modules likely to be affected.
new app/backend/test/error-envelope-coverage.spec.ts.

Difficulty. Medium
Estimated effort. M

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Backlog item #78 from `docs/maintainer-issue-backlog.md.

[Lansa-18]

Hi, I'd like to work on this @kilodesodiq-arch.

I'll add app/backend/test/error-envelope-coverage.spec.ts that parses the Swagger output, extracts every @ApiResponse with a 4xx or 5xx status, and asserts a corresponding test exists in error-handling.e2e-spec.ts for each one. Any undocumented code path fails CI loudly rather than silently passing. I'll also fill in any missing test cases surfaced by the matrix.

[grantfox-oss]

🦊 GrantFox — @Lansa-18 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #286)
2. Your PR will be reviewed by the ChainForgee maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Lansa-18's PR #323 was approved and merged by @kilodesodiq-arch.

🏆 @Lansa-18: You earned 40 FoxPoints for this contribution! Your current tier: Builder (1,324 total points). Track your full progress on GrantFox.

👏 Great work, @Lansa-18! Keep contributing to ChainForgee.