fix(sentry): keep scope validation alive when Sentry returns an empty error body

[L4XB]

Closes #6812.

What happened

SentryProvider.validate_scopes() called response.json() on every non-OK response. Sentry answers the deprecated /plugins/webhooks/ endpoint used for the project:write check with a 404 and an empty text/html body, so response.json() raised JSONDecodeError, the exception escaped validate_scopes, and the UI showed "Failed to revalidate scopes" with every scope on "Not checked" even though the provider was connected and pulling alerts.

Change

A small helper _scope_failure_detail(response) reads detail from a JSON body when there is one and otherwise falls back to HTTP <status>; the four failure sites (event:read with and without a configured project, project:read, project:write) use it. A single bad response now marks only its own scope, and a JSON error without detail reports the status instead of storing None.

Not changed: the /plugins/webhooks/ endpoint itself. Whether the project:write check should move to a non-legacy endpoint is a separate question; with this fix it degrades to "HTTP 404" for that scope instead of breaking the others.

Tests

New tests/test_sentry_provider.py builds a SentryProvider from a ProviderConfig and patches requests.get/requests.post with real requests.Response objects, so .ok and .json() behave as on the wire:

• 404 with an empty text/html body on the webhook POST → {"event:read": True, "project:read": True, "project:write": "HTTP 404"}
• 403 with a JSON detail → the detail text is reported as before
• 401 with JSON but no detail → "HTTP 401" for every scope

pytest tests/test_sentry_provider.py   # 3 passed
ruff check / ruff format --check (ruff 0.11.4 as pinned)   # clean

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[L4XB]

Closing as superseded: #6813 landed the production fix for #6812 first, and its _extract_error_detail covers everything my _scope_failure_detail did plus the non-dict JSON case. No reason for a second implementation.

The half that is still missing is the tests — #6813 merged without any, and there is no test file for this provider on main. I have reopened that part as #6821, rewritten against _extract_error_detail as merged rather than against my own version.