[DRAFT] Type sentry.snuba.metrics.query_builder and remove override

[cursor]

Summary

• remove sentry.snuba.metrics.query_builder from the mypy "sentry modules with typing issues" override list in pyproject.toml
• fix all discovered mypy issues in src/sentry/snuba/metrics/query_builder.py by tightening type narrowing, adding explicit assertions where unions need refinement, and using targeted casts for MetricOperationParams
• preserve runtime behavior while making the module pass strict type checking without module-level error-code suppression

Testing

• export PATH="$PWD/.venv/bin:$PATH" && .venv/bin/python -m tools.mypy_helpers.mypy_without_ignores src/sentry/snuba/metrics/query_builder.py
• export PATH="$PWD/.venv/bin:$PATH" && .venv/bin/mypy src/sentry/snuba/metrics/query_builder.py
• export PATH="$PWD/.venv/bin:$PATH" && .venv/bin/prek run -q

Environment-limited checks attempted

• make test-selective ❌ fails in this VM because gcloud is not installed
• .venv/bin/pytest -n3 -svv --reuse-db tests/sentry/snuba/metrics/test_query_builder.py ❌ fails due missing local Redis (localhost:6379 connection refused)
• SENTRY_SKIP_SERVICE_VALIDATION=1 retry still fails due missing Redis service

@armenzg could you please review this draft PR?

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.