[codex] Add /usr/bin/bash shell fallback

[pakrym-oai]

Why

Some Linux environments expose bash at /usr/bin/bash instead of /bin/bash. The shell detection fallback list should cover both standard locations once PATH/user-shell probing fails.

Stacked on #26480.

What changed

• Add /usr/bin/bash to the bash fallback path list in codex-shell-command.
• Extend shell type detection coverage for /usr/bin/bash.
• Add AGENTS.md testing guidance to avoid tests for statically defined values and negative tests for removed logic.

Verification

• just test -p codex-shell-command