Make probe result objects more immutable

Probe result objects (`FFmpegProbeResult`, `FFmpegStream`, `FFmpegFormat`, etc.) are currently mutable — fields are public and collections (e.g., streams list, tags map) are modifiable after construction.

We should make these objects more immutable to prevent accidental modification and improve thread safety.

This is part of the API modernization discussed in #316.

**Scope:**
- Return unmodifiable collections from getter methods (e.g., `Collections.unmodifiableList()` for streams)
- Consider making fields final where possible
- Ensure Gson deserialization still works (may need custom deserializers or builder pattern)

Depends on: #382, #383
Related: #316