Stabilize incomplete SSE retry test

[aibrahim-oai]

What changed

• The retry test now uses the same streaming SSE test server used by production-style tests instead of a wiremock sequence.
• The fixture is resolved via find_resource!, and the test asserts that exactly two outbound requests were sent.

Why this fixes the flake

• The old wiremock sequence approximated early-close behavior, but it did not reproduce the same streaming semantics the real client sees.
• That meant the retry path depended on mock implementation details instead of on the actual transport behavior we care about.
• Switching to the streaming SSE helper makes the test exercise the real early-close/retry contract, and counting requests directly verifies that we retried exactly once rather than merely hoping the sequence aligned.

Scope

• Test-only change.