feat: Stop reporting client errors to Sentry

[joamag]

Related to #38.

Client errors reach Sentry through every capture surface, the controller actions (request.exception), the REST dispatch (request.end) and the logging handler, even though they are caused by the client rather than by a failure of the server. In the last 7 days the LDJ instance submitted 3,345 events for requests answered with a 404 (eg: MVCRequestNotHandled for /favicon.ico), against 1,235 for server errors.

Changes

• is_ignored() also ignores the exceptions whose status code is a client error (400 to 499), as resolved by resolve_status_code() from the same status_code attribute that the MVC, REST and WSGI layers use to build the error response, so capture() drops them whatever the surface
• Messages logged without an exception are still reported, even for a request answered with a client error, as they are explicitly logged by the server
• The design document describes the behaviour

Verification

• Tests for every surface (controller actions, REST dispatch, logging handler and explicit capture), for the bounds of the range (399, 400, 499 and 500), for textual and invalid status codes, and for a message logged while a request is answered with a client error
• 5 of the new tests fail against the previous code and all of them pass with the change
• 100% coverage of the changed statements (4 of 4)
• Complete suite passing locally (21 bundles, 1248 tests) and black --check clean

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-14T19:31:32.129429Z	c4ab7af	PR opened

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between bef6904 and c4ab7af.

📒 Files selected for processing (4)

• CHANGELOG.md
• diagnostics/src/diagnostics_sentry/system.py
• diagnostics/src/diagnostics_sentry/test.py
• doc/design/004-sentry_error_reporting.md

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📝 Walkthrough

Walkthrough

The Sentry diagnostics now ignore exceptions with 4XX status codes across capture paths. Tests cover request, record, direct capture, and status-code handling. Messages without exceptions remain reportable.

Changes

Sentry client-error filtering

Layer / File(s)	Summary
Client-error filtering rule diagnostics/src/diagnostics_sentry/system.py, doc/design/004-sentry_error_reporting.md, CHANGELOG.md	is_ignored now ignores exceptions with status codes from 400 through 499. The design document and changelog describe the updated behavior.
Capture-surface validation diagnostics/src/diagnostics_sentry/test.py	Tests verify filtering through request, record, and direct capture paths. Tests also verify that messages without exceptions remain captured and that non-4XX values are not ignored.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to c4ab7

The change filters client-error exceptions while preserving explicitly logged messages, with no established merge-blocking risk.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 11.11% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 2 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: stopping Sentry reporting for client errors.
Description check	✅ Passed	The description directly explains the client-error filtering behavior, preserved message reporting, affected capture surfaces, and verification.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 11.11% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 2 files. (2 skipped: 2 unsupported.)

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[joamag]

LGTM