fix: avoid over-redacting ordinary urls

[mic1on]

Summary

• Keep ordinary single-segment URLs such as https://example.com/docs visible in redacted error messages.
• Continue redacting known provider path secrets and sensitive-looking custom single-segment notification paths.
• Add regression coverage for ordinary single-segment URLs and opaque custom token-like paths.

Closes #44

Test Coverage

• Added coverage that ordinary single-segment URLs remain unchanged.
• Extended custom base URL redaction coverage for token-like single-segment paths.

Test plan

• uv run --group dev pytest -q tests/test_notification.py -k redaction (2 passed)
• make lint
• make test (103 passed)
• make coverage (94% total coverage)
• uv build
• uv pip check