[codex] Fix native test determinism

[Stanzilla]

Summary

• shorten dispatch-only parity commands that were spending real default timeouts in browser wait handlers
• avoid navigating through auth_login in the parity coverage test by using a missing profile
• close the browser at the end of the parity coverage test
• guard AGENT_BROWSER_DEFAULT_TIMEOUT env mutation in timeout tests with EnvGuard

Root Cause

test_all_documented_actions_are_handled is a dispatch coverage test, but several minimal commands were exercising real wait paths with the default timeout. That made cargo test look hung and left Chrome alive while the test slowly timed out. Separately, default timeout tests mutated AGENT_BROWSER_DEFAULT_TIMEOUT without the shared env guard, so parallel test execution could leak the value between tests.

Validation

• cargo fmt -- --check
• git diff --check
• cargo test native::parity_tests::test_all_documented_actions_are_handled
• cargo test native::actions::tests::test_default_timeout_ms
• cargo test

[vercel]

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