Add proper getter methods to probe result objects

[bramp]

Currently, probe result objects (FFmpegProbeResult, FFmpegStream, FFmpegFormat, etc.) expose their data via public fields. We should add proper getter methods to follow Java conventions and enable future refactoring.

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

Scope:

• Add getter methods to FFmpegProbeResult, FFmpegStream, FFmpegFormat, and related probe result classes
• Keep existing public fields for backward compatibility (deprecation to follow separately)

Related: #316