feat(gemini): add thought signature preservation for thinking models

[dpolistwm]

Problem Statement

Issue #1199 - Gemini thinking models (like gemini-3-pro-preview and gemini-3-flash-preview) require thought_signature to be preserved for function calls in multi-turn conversations. Without this, the API returns:

400 Bad Request: Unable to submit request because function call <tool_name> 
in the N. content block is missing a thought_signature.

Additionally, when no tools are provided, the implementation was sending empty tool arrays causing:

400 Bad Request: tools[0].tool_type: required one_of 'tool_type' 
must have one initialized field

Solution

This PR implements proper thought signature preservation for Gemini thinking models:

1. Thought Signature Preservation: Captures thought_signature from function calls during streaming and includes it when sending function calls back to the API
2. Base64 Encoding: Uses base64 encoding/decoding for binary thought_signature data (instead of UTF-8 which fails on binary data)
3. Empty Tools Fix: Only includes tools parameter in the API request when there are actual tools to send
4. Backwards Compatibility: Converts toolUse without thoughtSignature to text representation to handle old session history gracefully

Changes Made

src/strands/models/gemini.py:

• Added base64 import for proper binary data encoding
• Modified _format_request_content_part() to:
  • Decode base64 thoughtSignature when sending reasoningContent
  • Decode base64 thoughtSignature when sending toolUse with signature
  • Convert toolUse without thoughtSignature to text representation
• Modified _format_request_tools() to only create Tool objects when tool_specs are provided
• Modified _format_request_config() to conditionally include tools only when non-empty
• Modified _format_chunk() to base64-encode thought_signature when capturing from streaming responses

tests/strands/models/test_gemini.py:

• Updated test_stream_request_with_reasoning to use valid base64 signature
• Updated test_stream_request_with_tool_use to include thoughtSignature
• Added test_stream_request_with_tool_use_no_thought_signature for backwards compatibility
• Updated test_stream_request_with_gemini_tools to expect correct tools format
• Updated test_stream_response_reasoning to expect base64-encoded signature

Testing

• ✅ All 33 existing Gemini model tests pass
• ✅ Added new test for toolUse without thoughtSignature
• ✅ Formatter and linter checks pass
• ✅ Tested with actual Gemini thinking models in production

Alignment with Development Tenets

• Simple at any scale: Handles both new thinking models and existing sessions transparently
• Extensible by design: Preserves existing functionality while adding new capability
• Composability: Works seamlessly with existing session management and conversation history
• The obvious path is the happy path: Automatic fallback for old data prevents breaking changes

Breaking Changes

None - this is backwards compatible. Old sessions without thoughtSignature will have tool calls converted to text representation, preserving conversation context.

Closes #1199