Normalize default-mode parsing and add regression test

[thb-hash]

Why

Environment and config values for the Ponytail default mode were not being normalized, so values with extra whitespace or unusual casing (for example, " FULL " or "\tultra\n") were ignored. This caused unexpected fallbacks or failures when users set defaults via env vars or config files.

What I changed

• Normalize PONYTAIL_DEFAULT_MODE and config.defaultMode through normalizeMode() in hooks/ponytail-config.js so whitespace and casing are handled, while still rejecting session-only modes like "review" as defaults.
• Add a focused regression test in pi-extension/test/helpers.test.js that verifies trimming and normalization for both env and config values.

Notes & testing

• Change is small and targeted: main logic moved to reuse existing normalizeMode() to avoid duplication and edge-case bugs.
• Ran unit tests locally: the helper tests pass (12/12) and full test suite run earlier passed.

Files changed

• hooks/ponytail-config.js — normalize env/config default parsing
• pi-extension/test/helpers.test.js — new regression test

Reviewers: small utility fix; extra attention appreciated on the normalization and config-path logic on Windows paths if needed.