Use the chat provider to apply transport-specific metadata cleanup.
Keep finish_reason removal in the common message conversion path.
Verify streaming and non-streaming HTTP payloads through LiteLLM,
metadata preservation for other providers, and source message immutability.
Remove response-only finish_reason metadata when converting stored messages
into subsequent model requests.
Strip provider_specific_fields from Groq messages and tool calls to avoid
strict request validation failures after a tool response. Preserve metadata
required by other providers, including Gemini thought signatures, without
mutating the original message entities.
Cover streaming and non-streaming request construction with regression tests.
Update _normalize_stream_tool_calls to preserve provider_specific_fields
(including thought_signature) from streaming tool call chunks. Also preserve
provider_specific_fields from delta in invoke_llm_stream.
This ensures Gemini's thought_signature is round-tripped correctly:
1. LiteLLM extracts thought_signature from Gemini response
2. It's preserved in Message/ToolCall entities (via SDK changes)
3. _convert_messages includes it in the next request
Also add unit tests for provider_specific_fields round-tripping.
Fixes: langbot-app/LangBot#1899
BiFangKNT