feat: Add streaming support for batched prompts

[XBastille]

Description

Adds streaming support for batched prompts, resolving issue #406.

Changes

• Removed restriction that blocked batched streaming in _sampler.py
• Fixed _stream_sample_loop to wait for ALL batch elements to complete (not just first)
• Updated _stream_decode_state to properly handle batch dimensions
• Net reduction of 3 lines of code

Testing

• Tested with Gemma3_4B on NVIDIA L40s GPU
• Single prompt streaming: Works (unchanged)
• Batched non-streaming: Works (unchanged)
• Batched streaming: Now works (new feature!)

Backward Compatibility

Fully backward compatible - no breaking changes to existing API.

Fixes #406