fix(auth): retry spawn on ETXTBSY in the auth-status check

[benw5483]

Summary

• Route both spawn sites in the claude auth status check through the existing spawn_with_etxtbsy_retry helper, so a freshly written executable no longer fails the exec with ETXTBSY ("text file busy").
• Share one spawn_auth_child function between check_auth_async and check_auth_async_no_json, which previously duplicated the spawn and its error mapping verbatim.

Why

The runner spawn paths already retry on ETXTBSY; the auth-status check was the one that still called Command::spawn() directly. That gap surfaced as an intermittent failed to spawn claude: Text file busy (os error 26) panic in test_check_auth_async_success under the instrumented coverage build, which runs slowly enough to lose a race the plain Test job usually wins. A single panic there aborts lcov.info generation, so Coverage Enforcement fails with a missing-report error rather than a real per-file miss.

probe_claude_auth_async runs the same claude auth status --json command and was already covered, so this brings the two into line.

Test plan

• cargo test --workspace --features integration — 3103 passed, 0 failed in the main crate; all nine integration suites green.
• cargo fmt --check and cargo clippy -- -D warnings — both clean.
• cargo llvm-cov reproducing the coverage job — cli/commands/auth.rs 552/552 lines and runner/util.rs 200/200, with every file in the tree at 100%.

The retry branch itself is already covered by the four unit tests on spawn_with_etxtbsy_retry in src/runner/util.rs: first-attempt success, retry-then-succeed, non-ETXTBSY error not retried, and retries exhausted. No new integration test provokes a real ETXTBSY, because it cannot be reproduced portably. Holding a writable file descriptor on a fixture script raises no error on macOS, so such a test would be Linux-only and timing-dependent, which is the opposite of what this change is for.

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Generated by the operator's software factory.
• City: factory-main · Agent: local-core.builder-1
• On behalf of: @benw5483