ci: stabilize readiness retry test

[StevenWang-CY]

Summary

• replace the retry test delayed listener and wake-up cleanup with two deterministic URLError failures followed by a real /v1/models success through the shared server fixture
• assert exactly three attempts and one actual server request, preserving the retry contract without port-reservation or listener-shutdown races
• leave production code and behavior unchanged

On unmodified main at 661d8db, the Python 3.13 job completed the readiness request and then failed in test cleanup when its wake-up connection raced the listener closing: ConnectionRefusedError with 113 tests already passing. Python 3.10-3.12 passed in the same matrix. A 200-run local stress control on the untouched test did not reproduce the scheduling-dependent failure.

Test Plan

• focused Python 3.13 regression repeated 50 times: 50 passed
• CI-style Python 3.13 wheel build, install, full suite, and wheel validation: 114 passed; wheel validation passed
• full editable Python 3.13 suite: 111 passed, 3 skipped
• cargo fmt --all -- --check
• cargo check --workspace --all-targets --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features (passed; 5 PostgreSQL tests ignored locally without TEST_POSTGRES_URL)
• launcher contracts and CLI end-to-end tests
• cassette validation: 108/108 cassettes, 238 turns
• strict MkDocs build

Local repository-wide pre-commit hooks passed except that the platform-hanging check-added-large-files all-files invocation was skipped; that hook passed directly against the only changed file.

Rebased the unchanged patch onto current main at c04ac196. All 12 hosted checks pass on head 6455cae, including Python 3.10-3.13, Rust, PostgreSQL, both CLI harnesses, pre-commit, and DCO.

[StevenWang-CY]

@franciscojavierarceo could you review this? All 12 checks pass after rebasing onto main. This one-file test fix removes the cleanup race while retaining a real HTTP success check; production code is unchanged.