feat(mac): Fix extension startup failures from missing deps; add safe cross-platform auto-repair for known modules

[deya-eldeen]

Summary

This PR fixes model extension startup crashes when an extension venv is present but incomplete (for example, ModuleNotFoundError: No module named 'PIL').

It adds a conservative auto-repair step in the extension subprocess startup path:

• Detect missing-module errors from runner startup.
• Auto-install only explicitly allowed module mappings (currently PIL -> Pillow).
• Retry startup after successful install.

Why

Some extension setups can leave a partially initialized venv, causing startup to fail before the extension can report as ready.
Previously, this resulted in runtime errors like:

• Expected 'ready', got: ... Failed to load generator class
• ModuleNotFoundError: No module named 'PIL'

Changes

• Updated api/services/extension_process.py:

1. Added missing-module parsing from startup error payloads.
2. Added safe mapping-based auto-repair (_AUTO_REPAIR_PACKAGE_MAP).
3. Added startup retry loop (up to 3 attempts total).
4. Added explicit error reporting when auto-repair install fails.

Safety / Compatibility

• Cross-platform compatible (macOS/Linux/Windows): uses the extension’s own venv Python (python -m pip install ...).
• Conservative behavior: no arbitrary package-name guessing; only known mappings are auto-installed.
• If mapping is unknown, behavior falls back to normal startup error (no hidden installs).

Validation

• Python syntax check passed:
  • python3 -m py_compile api/services/extension_process.py
• Manual verification confirmed extension runner can reach ready after installing missing Pillow.