Fix Whisper ASR timestamp regression

[anshusaurav]

Summary

Fixes #1684 — Whisper ASR timestamp regression between v3.8.1 and v4.2.0.

Three fixes targeting the seek loop and logits processor introduced in commit 43b2662 ("Overdue Whisper fixes #1594"):

• Use actual audio length for seek loop bounds (modeling_whisper.js): The seek loop used input_features.dims[2] (always 3000 = padded 30s) as total_frames instead of generation_config.num_frames (set by the ASR pipeline to the real mel frame count). For clips shorter than 30 seconds, this caused the loop to process silence/padding as real audio, producing hallucinated text and incorrect timestamps.

• Apply max_initial_timestamp_index constraint (logits_process.js): A continue statement in WhisperTimeStampLogitsProcessor._call() skipped the max_initial_timestamp_index check at line 326, making it dead code. This allowed the model to generate any timestamp as its first token instead of being constrained to the allowed range. The Python reference implementation has no such early exit — both blocks execute sequentially.

• Guard against infinite seek loop (modeling_whisper.js): If segment_offset computes to zero (e.g., from a degenerate timestamp pair), the seek loop would run forever. Added a break when segment_offset <= 0.

Test plan

• Verify with the reproduction case from ASR (Whisper) regression for detected timestamps #1684 (Steve Jobs keynote audio) — timestamps should match v3.8.1 output
• Verify [applause] segment is no longer dropped
• Existing Whisper prefix token tests (test_modeling_whisper.js) still pass — they use max_new_tokens which bypasses the seek loop
• Test with audio shorter than 30s — seek loop should terminate after one iteration
• Test with audio longer than 30s — multi-segment seek should still work correctly