fix(snuba): distinguish upstream proxy errors from snuba errors

[JoshFerge]

Summary

When envoy (or another upstream proxy) returns a 504 with body upstream request timeout before the request reaches snuba, _bulk_snuba_query fails to parse it as JSON and raises a generic SnubaError("Failed to parse snuba error response"). This obscures the real failure mode — the request never reached snuba, so:

• there are no snuba traces for the failed query
• the actual cause lives in the proxy's response body, not snuba logs
• all proxy 5xx flavors get bucketed under the same opaque error
• the API layer can't tell it's a timeout, so the frontend gets a generic 500 instead of a 504

Investigation context: issue 6969043325 is dominated by 504s from envoy in front of snuba-api, almost all with the literal body upstream request timeout.

This change introduces two new exception classes in src/sentry/utils/snuba.py:

• InvalidSnubaResponseError — raised when a non-2xx response can't be decoded as JSON. Captures status and body for debugging.
• SnubaUpstreamRequestTimeout — subclass for 504 responses, or any status with the envoy-default body upstream request timeout.

Both subclass SnubaError, so existing handlers that catch SnubaError continue to work — this only refines what callers can choose to catch.

It also wires SnubaUpstreamRequestTimeout into handle_query_errors in src/sentry/api/utils.py alongside QueryExecutionTimeMaximum / ReadTimeoutError, so it surfaces as the existing TimeoutException (HTTP 504) — the frontend already knows what to do with that.

Test plan

• New unit tests in tests/sentry/utils/test_snuba.py::SnubaInvalidJsonResponseTest cover:
  • 504 with upstream request timeout body → SnubaUpstreamRequestTimeout
  • non-504 status with the same body → SnubaUpstreamRequestTimeout (defensive)
  • other non-JSON 5xx (e.g. 502 no healthy upstream) → InvalidSnubaResponseError only
• New test in tests/sentry/api/test_utils.py::HandleQueryErrorsTest::test_handle_snuba_upstream_request_timeout confirms SnubaUpstreamRequestTimeout is converted to TimeoutException by handle_query_errors
• Full tests/sentry/utils/test_snuba.py and tests/sentry/api/test_utils.py::HandleQueryErrorsTest pass
• prek run -q passes on touched files