fix(seer): Handle non-2xx HTTP responses from severity-score endpoint

[sentry]

This PR fixes a JSONDecodeError that occurred when the Seer /v0/issues/severity-score endpoint returned a non-2xx HTTP status code (e.g., 503 Service Unavailable).

Previously, the _get_severity_score() function in src/sentry/event_manager.py would attempt to parse response.data with orjson.loads() immediately after make_severity_score_request(), without first checking the HTTP status code. When Seer returned an error status, the response body often contained an HTML error page, leading to a JSONDecodeError.

The fix introduces a check for response.status >= 400 before attempting JSON parsing. If an error status is detected, an Exception is raised. This exception is then caught by the existing except Exception block, which gracefully handles the microservice error by setting the reason to microservice_error and incrementing relevant metrics, thus preventing the JSONDecodeError.

Additionally, a new metric tag microservice_http_error is added to distinguish these specific HTTP errors from other unknown microservice errors. A new test case has also been added to tests/sentry/event_manager/test_severity.py to cover this scenario.

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Fixes SENTRY-5M0K

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