[lesnik512]

Changes

• Share HTTP metrics cases across FastAPI, Litestar, and FastMCP and assert exact request counts for OpenTelemetry and Prometheus.
• Test probe failures and warnings on the health-check instrument while keeping HTTP 200 and 503 coverage for each framework.
• Synchronize the concurrency test with events so it does not depend on elapsed time.

Verification

• just test-ci passed with 658 tests and 100% line coverage. Ruff and ty check passed.
• The HTTP metrics matrix passed with both legacy and stable OpenTelemetry metric names active.
• A temporary second request made the exact-count assertions fail for both backends.