fix(testing): accept pytest flags without explicit separator

[xushun007]

Fixes #42189

Summary

Teach the per-file test runner to accept pytest passthrough flags even when the caller omits the explicit -- separator.

This preserves the documented -- behavior while also supporting the common shorthand:

scripts/run_tests.sh tests/test_run_tests_parallel.py -q

## What changed

- added _split_runner_and_pytest_args() to preserve explicit -- handling
- fall back to parse_known_args() when no separator is present
- added regression tests for both separatorless and explicit-separator forms

## Validation

- ./.venv/bin/python -m pytest tests/test_run_tests_parallel.py -q -o addopts=
- scripts/run_tests.sh tests/test_run_tests_parallel.py -q -o addopts=

[teknium1]

Thanks for addressing the test-runner ergonomics.

Problems

• The runner portion is already implemented by merged PR fix(tests): bare pytest flags pass through run_tests.sh without a '--' separator #54008 (252391768): scripts/run_tests_parallel.py:670-732 routes bare pytest flags while preserving runner options and value-taking flags; tests/test_run_tests_parallel.py:228-279 covers bare -q, -k values, explicit --, and positional paths.
• This PR also contains commit e061904d, an independent auxiliary 5xx-fallback change not described in the PR summary. That work maps to still-open refactor(auxiliary): separate _is_server_error and _is_unavailable_error from _is_payment_error #42088; current main has a newer transient-retry path (agent/auxiliary_client.py:2987-3003) and both fallback gates (:6866-6896, :7383-7405) need a focused current-main design review.
• The auxiliary diff changes both sync and async paths, but only adds a sync regression test.

Suggested changes

• Please treat the test-runner portion as superseded by 252391768.
• If pursuing the auxiliary change, split and rebase that work around refactor(auxiliary): separate _is_server_error and _is_unavailable_error from _is_payment_error #42088 with sync and async coverage.

Automated hermes-sweeper review.