Claude's extended thinking feature generates thought_signature fields
(message IDs like "msg_abc123...") in content blocks. When these are
sent to Google's Gemini API, it expects Base64-encoded bytes and
rejects Claude's format with a 400 error.
This commit adds stripThoughtSignatures() to remove these fields from
assistant message content blocks during sanitization, enabling session
histories to be shared across different providers (e.g., Claude → Gemini).
Fixes cross-provider session bug where switching from Claude-thinking
to Gemini (or vice versa) would fail with:
"Invalid value at 'thought_signature' (TYPE_BYTES), Base64 decoding failed"
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
c4e8b60d2c