fix: preserve reasoning content signature from LiteLLM thinking models

[giulio-leone]

Problem

When streaming responses from thinking models (e.g., Gemini) via LiteLLM, the reasoning content signature is silently dropped. The LiteLLM adapter's _process_choice_content only emits reasoning text from delta.reasoning_content but never captures the signature from delta.thinking.signature.

The event loop in streaming.py already correctly handles reasoningContent.signature deltas (lines 240-248, 314-315), but the LiteLLM model provider never emits them.

Root Cause

LiteLLM provides reasoning signatures via a separate thinking attribute on stream deltas (delta.thinking.signature), distinct from delta.reasoning_content (which only contains the text). The LiteLLM model provider's _process_choice_content method only processed reasoning_content text, never checking for the thinking.signature attribute.

Fix

Added a check in _process_choice_content() for the thinking.signature attribute on each streaming delta. When a valid string signature is found, a contentBlockDelta with reasoningContent.signature is emitted so the event loop can accumulate and store it in the final content block.

An isinstance(sig, str) guard prevents false positives from unittest.mock.Mock objects that auto-create attributes.

Testing

• Added test_stream_preserves_thinking_signature that simulates a 3-chunk streaming response with reasoning text and signature
• All 42 existing tests pass with 0 errors/0 warnings

Fixes #1764

[giulio-leone]

All CI checks pass. Ready for review.