fix: preserve Gemini thought_signature in LiteLLM multi-turn tool calls

[giulio-leone]

Issue

Closes #1764

Description

When using Gemini thinking models (e.g., gemini-2.5-flash, gemini-3-flash) through the LiteLLM model provider, multi-turn conversations with tool calls fail with:

function call 'current_time' in the 2. content block is missing a thought_signature

Root Cause

Gemini thinking models require a thought_signature on each function call in multi-turn conversations. LiteLLM encodes this signature into the tool call ID using a __thought__ separator (e.g., call_abc__thought__base64sig). However, Strands' OpenAIModel.format_chunk() passes the encoded ID through without extracting the signature into the reasoningSignature field that the streaming layer already supports.

While the encoded ID survives the round-trip in simple cases, the signature must also be explicitly stored in reasoningSignature so that:

1. The streaming infrastructure can properly track it through content block lifecycle
2. The signature can be re-encoded into the tool call ID even when the ID is cleaned or regenerated
3. provider_specific_fields.thought_signature (an alternative LiteLLM transport) is also captured

Changes

src/strands/models/litellm.py:

• Override format_chunk to detect __thought__ in tool call IDs and/or provider_specific_fields.thought_signature, extracting the signature into reasoningSignature on the contentBlockStart event
• Override format_request_message_tool_call to re-encode reasoningSignature back into the tool call ID when not already present, ensuring LiteLLM can reconstruct the Gemini-native format

tests/strands/models/test_litellm.py:

• 7 new unit tests covering: extraction from encoded ID, extraction from provider_specific_fields, no-signature passthrough, signature encoding, double-encode prevention, and full round-trip preservation

Checklist

• I've read CONTRIBUTING.md
• I've updated or added tests to cover changes
• I've verified all existing tests pass (48/48)
• This change is backward compatible (no breaking changes)

[giulio-leone]

Friendly ping — preserves the Gemini thought_signature field through LiteLLM multi-turn tool call cycles, fixing broken thinking chains.