test(streaming): cover partial JSON on max_tokens

[wilyan09007]

Summary:
Refs #794 by adding regression coverage for streaming tool-use input when max_tokens stops an incomplete JSON stream. The tests verify current behavior preserves partial JSON snapshots and parsed input.

Changes:

• Add sync streaming coverage for incomplete partial JSON on max_tokens.
• Add async streaming coverage for incomplete partial JSON on max_tokens.
• Add beta streaming coverage for the same fixture.
• Assert partial input_json events remain available.
• Assert the final tool-use snapshot preserves parsed partial input.

Testing instructions:

• Run uv run pytest tests\lib\streaming\test_messages.py tests\lib\streaming\test_beta_messages.py tests\lib\streaming\test_partial_json.py -q.
• Run uv run ruff check tests\lib\streaming\test_messages.py tests\lib\streaming\test_beta_messages.py.
• Run uv run ruff format --check tests\lib\streaming\test_messages.py tests\lib\streaming\test_beta_messages.py.
• Confirm the incomplete JSON fixture still produces input_json events.