feat[vLLM x v5]: Expose max_source_positions on VibeVoiceAsrConfig
Author: harshaljanjani
Pull request: yes

### What does this PR do?

→ Fixes https://github.com/vllm-project/vllm/pull/39330#discussion_r3364359475
→ Exposes `max_source_positions` on `VibeVoiceAsrConfig` so profiling can resolve the model's max audio token budget.
→ Companion to the [vLLM Transformers audio backend PR](https://github.com/vllm-project/vllm/pull/39330). vLLM's `get_max_audio_tokens` looks for one of `max_source_positions`, `max_position_embeddings` or `max_pos_emb` and  VibeVoice exposes none of these (its acoustic tokenizer is ConvNet-based with no positional embeddings) so profiling raises `ValueError`. With this change the same path resolves cleanly to `450` (`max_source_positions = ceil(1440000 (60s @ 24kHz) / 3200) = 450`, which is already in the config, is now exposed through the property).
→ Made sure this doesn't cause any regressions in `tests/models/vibevoice_asr/`.

cc: @eustlb @vasqu

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Comments:
@harshaljanjani thanks for pointing this out!

As you've suggested, I think we should expose this attribute but I would do it differently to be consistent with existing models:
- as an attribute (rather than property)
- `max_position_embeddings` as it is seems to be more frequent than `max_source_positions`? and moving forward we should probably stick with this `max_position_embeddings` @eustlb?
- like other models, add it to the [encoder config](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/vibevoice_acoustic_tokenizer/configuration_vibevoice_acoustic_tokenizer.py#L92) of VibeVoice rather than its main modeling config

for reference, I've listed below the attributes that existing models expose:
- audioflamingo3 / musicflamingo: [max_source_positions](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/audioflamingo3/configuration_audioflamingo3.py#L68)
- voxtral / voxtral_realtime: [max_position_embeddings](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/voxtral_realtime/configuration_voxtral_realtime.py#L111) 
- glm asr: [max_position_embeddings](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/glmasr/configuration_glmasr.py#L50)
- parakeet: [max_position_embeddings](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/parakeet/configuration_parakeet.py#L80)
- cohere: [max_position_embeddings](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/cohere_asr/configuration_cohere_asr.py#L72)
- ibm: [max_pos_emb](https://github.com/huggingface/transformers/blob/1b7bc25c46775af67c340478b01fedc70dfcc33e/src/transformers/models/granite_speech_plus/configuration_granite_speech_plus.py#L80)

Good day @vasqu and @ebezzam! Just checking in to see if there have been any updates :)

Done, thank you both for your time :)

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: vibevoice_asr

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