feat(backend): Add language fallback for YouTube transcription block (#11057)

## Problem

The YouTube transcription block would fail when attempting to transcribe
videos that only had transcripts available in non-English languages.
Even when usable transcripts existed in other languages, the block would
raise a `NoTranscriptFound` error because it only requested English
transcripts.

**Example video that would fail:**
https://www.youtube.com/watch?v=3AMl5d2NKpQ (only has Hungarian
transcripts)

**Error message:**
```
Could not retrieve a transcript for the video https://www.youtube.com/watch?v=3AMl5d2NKpQ! 
No transcripts were found for any of the requested language codes: ('en',)

For this video (3AMl5d2NKpQ) transcripts are available in the following languages:
(GENERATED) - hu ("Hungarian (auto-generated)")
```

## Solution

Implemented intelligent language fallback in the
`TranscribeYoutubeVideoBlock.get_transcript()` method:

1. **First**, tries to fetch English transcript (maintains backward
compatibility)
2. **If English unavailable**, lists all available transcripts and
selects the first one using this priority:
   - Manually created transcripts (any language)
   - Auto-generated transcripts (any language)
3. **Only fails** if no transcripts exist at all

**Example behavior:**
```python
# Before: Video with only Hungarian transcript
get_transcript("3AMl5d2NKpQ")  # ❌ Raises NoTranscriptFound

# After: Video with only Hungarian transcript  
get_transcript("3AMl5d2NKpQ")  # ✅ Returns Hungarian transcript
```

## Changes

- **Modified** `backend/blocks/youtube.py`: Added try-catch logic to
fallback to any available language when English is not found
- **Added** `test/blocks/test_youtube.py`: Comprehensive test suite
covering URL extraction, language fallback, transcript preferences, and
error handling (7 tests)
- **Updated** `docs/content/platform/blocks/youtube.md`: Documented the
language fallback behavior and transcript priority order

## Testing

- ✅ All 7 new unit tests pass
- ✅ Block integration test passes
- ✅ Full test suite: 621 passed, 0 failed (no regressions)
- ✅ Code formatting and linting pass

## Impact

This fix enables the YouTube transcription block to work with
international content while maintaining full backward compatibility:

- ✅ Videos in any language can now be transcribed
- ✅ English is still preferred when available
- ✅ No breaking changes to existing functionality
- ✅ Graceful degradation to available languages

Fixes #10637
Fixes https://linear.app/autogpt/issue/OPEN-2626

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<details>

<summary>Original prompt</summary>

> Issue Title: if theres only one lanague available for transcribe
youtube return that langage not an error
> Issue Description: `Could not retrieve a transcript for the video
https://www.youtube.com/watch?v=3AMl5d2NKpQ! This is most likely caused
by: No transcripts were found for any of the requested language codes:
('en',) For this video (3AMl5d2NKpQ) transcripts are available in the
following languages: (MANUALLY CREATED) None (GENERATED) - hu
("Hungarian (auto-generated)") (TRANSLATION LANGUAGES) None If you are
sure that the described cause is not responsible for this error and that
a transcript should be retrievable, please create an issue at
https://github.com/jdepoix/youtube-transcript-api/issues. Please add
which version of youtube_transcript_api you are using and provide the
information needed to replicate the error. Also make sure that there are
no open issues which already describe your problem!` you can use this
video to test:
[https://www.youtube.com/watch?v=3AMl5d2NKpQ\`](https://www.youtube.com/watch?v=3AMl5d2NKpQ%60)
> Fixes
https://linear.app/autogpt/issue/OPEN-2626/if-theres-only-one-lanague-available-for-transcribe-youtube-return
> 
> 
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> Comment by User :
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issue](https://github.com/Significant-Gravitas/AutoGPT/issues/10637).
All replies are displayed in both locations.
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Co-authored-by: Nicholas Tindle <nicholas.tindle@agpt.co>

docs/content/platform/blocks/youtube.md

@@ -7,7 +7,7 @@ A block that transcribes the audio content of a YouTube video into text.
 This block takes a YouTube video URL as input and produces a text transcript of the video's audio content. It also extracts and provides the unique video ID associated with the YouTube video.
 
 ### How it works
-The block first extracts the video ID from the provided YouTube URL. It then uses this ID to fetch the video's transcript. The transcript is processed and formatted into a readable text format. If any errors occur during this process, the block will capture and report them.
+The block first extracts the video ID from the provided YouTube URL. It then uses this ID to fetch the video's transcript, preferring English when available. If an English transcript is not available, the block will automatically use the first available transcript in any other language (prioritizing manually created transcripts over auto-generated ones). The transcript is processed and formatted into a readable text format. If any errors occur during this process, the block will capture and report them.
 
 ### Inputs
 | Input | Description |
@@ -22,5 +22,5 @@ The block first extracts the video ID from the provided YouTube URL. It then use
 | Error | Any error message that occurs if the transcription process fails. |
 
 ### Possible use case
-A content creator could use this block to automatically generate subtitles for their YouTube videos. They could also use it to create text-based summaries of video content for SEO purposes or to make their content more accessible to hearing-impaired viewers.
+A content creator could use this block to automatically generate subtitles for their YouTube videos. They could also use it to create text-based summaries of video content for SEO purposes or to make their content more accessible to hearing-impaired viewers. The automatic language fallback feature ensures that transcripts can be obtained even from videos that only have subtitles in non-English languages.