test(v2/featureflag): add integration tests for timeout fallback behaviour

What this MR does

Adds v2/featureflag/timeout_test.go with two integration tests that verify the HTTP timeout fallback path end-to-end:

• TestBooleanValueDetails_ReturnsFallbackOnServerTimeout — starts an httptest.Server that accepts connections but never writes a response (simulating a hung Flipt service), creates a client with a 100 ms timeout, calls BooleanValueDetails, and asserts:
  • the default flag value is returned
  • an error is returned (for caller observability)
  • evaluation completes within timeout × 10 (does not hang)
  • the OTel span records the failure
• TestStringValueDetails_ReturnsFallbackOnServerTimeout — same pattern for StringValueDetails

Why

Closes criterion 5 of team-tasks#4382: "Behaviour is validated under simulated slow/unresponsive feature flag service conditions".

The existing transport_test.go unit-tests resolveHTTPClient in isolation. These new tests provide end-to-end confidence that the 2 s default timeout propagates correctly through the Flipt provider → HTTP client → caller path.

How to test

cd v2 && go test ./featureflag/ -run "Timeout" -v

All 40 tests in the package pass.

Checklist

• Tests added
• All existing tests pass (go test ./featureflag/...)
• No production code changes