linux-rust: refresh the audio card index on activation

[mmatczuk]

Summary

• resolve the current PulseAudio/PipeWire card by Bluetooth MAC before every A2DP activation
• update the cached card index used by subsequent profile operations
• stop cleanly when the current card cannot be found instead of using a stale index

Validation

• cargo check --manifest-path linux-rust/Cargo.toml
• cargo nextest run --manifest-path linux-rust/Cargo.toml --no-tests pass

Fixes #681