fix string building for onNewSegments and getTextSegments

[jiangts]

This PR fixes crashes in the iOS implementation onNewSegments. Tested on 0.4.3 tag.

Key Changes

• Improved string handling:

  • Replaced repeated NSString concatenation with a NSMutableString builder.
  • Added UTF-8 decoding with a Latin-1 fallback to handle invalid/“dirty” input without returning nil.
  • Guaranteed all dictionary values are non-nil (empty string fallback).

• onNewSegments callback

  • Added @autoreleasepool to reduce memory pressure inside callback loops.
  • Moved rnwhisper_segments_callback_data allocation to the heap.
  • Introduced isHeapAllocated flag and cleanup_callback_data helper to prevent leaks and use-after-free.

• getTextSegments

  • Applied the same safe string handling improvements as above.
  • Ensured returned result and segments are always consistent and non-nil.

Why

• Prevents crashes from invalid UTF-8 sequences.
• Avoids inserting nil values into dictionaries.
• Eliminates risk of dangling pointers when callbacks outlive stack variables.
• Improves memory safety and stability for real-time transcription.

[derryc09]

Thanks for this, I've been debugging this for the past week.
Whisper.rn causes the app to crash when transcribing, in my case, Japanese.

[jhen0409]

Thanks for the PR! It would be helpful if you could provide a valid test file.

Also, note that we will replace all native iOS/Android implementations with RNWhisperJSI in the future, that still decode text segments as utf8.

[abretonc7s]

Can confirm this bug is still present in 0.6.0-rc.0. Calling transcribeData with onNewSegments on iOS crashes with EXC_BAD_ACCESS at RNWhisperContext.mm:463.

Root cause: the onNewSegments ObjC block is stored in the C struct without an explicit [copy], so ARC does not retain it. Heap-allocating the struct and using [onNewSegments copy] fixes it:

struct rnwhisper_segments_callback_data *user_data = new rnwhisper_segments_callback_data();
user_data->onNewSegments = [onNewSegments copy];
user_data->tdrzEnable = options[@"tdrzEnable"] && [options[@"tdrzEnable"] boolValue];
user_data->total_n_new = 0;
params.new_segment_callback_user_data = user_data;

// after fullTranscribe returns:
if (params.new_segment_callback_user_data) {
    delete (struct rnwhisper_segments_callback_data *)params.new_segment_callback_user_data;
    params.new_segment_callback_user_data = nullptr;
}

The JFK sample audio from the whisper.rn example app reliably reproduces the crash and could serve as the test file requested above.