test(cortex-plugins): add comprehensive edge case tests

Add tests covering:
- Malformed manifests (empty ID, invalid semver, bad TOML syntax)
- Invalid characters in plugin ID
- Permission and hook execution edge cases
- Hook priority ordering and abort/skip/continue behavior
- Registry SSRF protection (private IPs, metadata endpoints)
- Directory traversal protection
- Plugin registration/unregistration lifecycle
- Configuration edge cases (enable/disable, whitelist mode)
- Error handling and display messages
- Plugin state transitions
- Remote registry management
- Signing and checksum verification

59 tests pass covering error handling and boundary conditions.

Author: echobt