Add edge-case tests for safe_filename() normalization

[xiaoleiy]

Good first issue (tests only, no risk). Strengthen coverage for core.safe_filename, which makes
downloaded filenames cloud-/filesystem-safe.

Add cases to tests/test_core.py::test_safe_filename for:

• very long mixed CJK+ASCII titles (result stays ≤ 120 chars and non-empty),
• titles that reduce to empty after stripping (→ "untitled"),
• more Windows reserved names: lowercase com1, LPT9, CON with a trailing space,
• titles made up entirely of emoji/symbols.

Where: logic in src/podpull/core.py; tests in tests/test_core.py.
Acceptance: new assertions pass. If you find a real gap in safe_filename, note it here before changing it.

[syed7-crypto]

Hi there! I'd love to work on this issue as my first Python contribution. Could you please assign it to me?

[xiaoleiy]

Hi there! I'd love to work on this issue as my first Python contribution. Could you please assign it to me?

Sure, appreciate your help on this!