Respect caller timestamp options in batched transcribe

[ponpaku]

Summary

This PR restores caller-provided condition_on_previous_text and max_initial_timestamp in BatchedInferencePipeline.transcribe().

Related issue: #1427

Previously, the batched path hardcoded these values instead of using the caller request, making it behaviorally non-equivalent to WhisperModel.transcribe() for timestamped decoding.

What this PR changes

• pass through condition_on_previous_text in batched transcription options
• pass through max_initial_timestamp in batched transcription options
• add regression tests that verify both values are preserved in batched transcription options

What this PR does not try to solve

This PR is intentionally narrow.

It does not attempt to fix all direct/batched quality gaps such as:

• prompt/history update design
• fallback behavior differences
• VAD chunk packing strategy

Validation

• added lightweight regression tests for option propagation
• verified the new tests pass locally with pytest
• confirmed on a public FLEURS Japanese fixture that upstream batched transcription reports the overridden values, while the patched branch preserves the caller values