fix: preserve Anthropic thinking blocks and signatures in LiteLLM round-trip

[giulio-leone]

Summary

Fixes #4801 — Adaptive thinking is broken when using Claude models through LiteLLM.

Root Cause

When Claude produces extended thinking with thinking_blocks (each containing a type, thinking text, and signature), the round-trip through ADK's LiteLLM integration silently loses them:

1. _extract_reasoning_value() only read reasoning_content (a flattened string without signatures), ignoring the richer thinking_blocks field
2. _content_to_message_param() set reasoning_content on the outgoing ChatCompletionAssistantMessage, but LiteLLM's anthropic_messages_pt() prompt template silently drops the reasoning_content field entirely
3. Result: thinking blocks vanish from conversation history after turn 1; Claude stops producing them

Fix

Three coordinated changes in lite_llm.py:

Change	What it does
_is_anthropic_provider() helper	Detects anthropic, bedrock, vertex_ai providers
_extract_reasoning_value()	Now prefers thinking_blocks (with per-block signatures) over reasoning_content
_convert_reasoning_value_to_parts()	Handles ChatCompletionThinkingBlock dicts, preserving thought_signature
_content_to_message_param()	For Anthropic providers, embeds thinking blocks directly in the message content list as {"type": "thinking", ...} dicts — this format passes through LiteLLM's anthropic_messages_pt() correctly

For non-Anthropic providers (OpenAI, etc.), behavior is unchanged — reasoning_content is still used.

Verification

• LiteLLM's anthropic_messages_pt() was tested to confirm:
  • reasoning_content field → DROPPED (existing LiteLLM bug)
  • content as list with {"type": "thinking", ...} → PRESERVED ✅
  • Signatures in thinking blocks → PRESERVED when in content list ✅

Tests

Added 7 targeted tests covering:

• _is_anthropic_provider() — provider detection
• _extract_reasoning_value() — prefers thinking_blocks over reasoning_content
• _convert_reasoning_value_to_parts() — signature preservation from block dicts
• _convert_reasoning_value_to_parts() — plain string fallback (no signature)
• _content_to_message_param() — Anthropic: thinking blocks embedded in content list
• _content_to_message_param() — OpenAI: reasoning_content field used (unchanged)
• _content_to_message_param() — Anthropic thinking + tool calls combined

Full test suite: 4732 passed, 0 failures

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Thanks!

[rohityan]

Hi @giulio-leone , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!

[giulio-leone]

@rohityan Thanks for the heads-up! I'll get the Google CLA signed. Will follow up once it's done.

[giulio-leone]

I have signed the Google CLA. Could you please re-run the CLA check? Thank you!

[giulio-leone]

Hi @rohityan — the CLA is now signed and passing ✅ (it was a Co-authored-by trailer issue that has been resolved). Ready for review!