AnthropicLlm.part_to_message_block() drops non-standard FunctionResponse dicts (breaks SkillToolset with Claude)` #4779

Describe the Bug:
AnthropicLlm.part_to_message_block() only handles FunctionResponse.response dicts that contain a "content" or "result" key. When SkillToolset.load_skill() returns a response with keys skill_name, instructions, frontmatter, the method falls through and returns an empty string. This means Claude models never see the skill instructions after calling load_skill.

Steps to Reproduce:

1. Create an agent with AnthropicLlm (any Claude model)
2. Add a SkillToolset with at least one skill
3. Ask the agent to use the skill
4. The agent calls load_skill but receives an empty tool response — it cannot follow the skill instructions

Expected Behavior:
The tool response should contain the full skill instructions returned by load_skill.

Observed Behavior:
The tool response is empty. The agent sees no skill instructions.

Root cause

In anthropic_llm.py, part_to_message_block() checks for response.get("content") and response.get("result") but doesn't handle other dict structures. The load_skill tool returns {"skill_name": ..., "instructions": ..., "frontmatter": ...} which matches neither key.

Environment Details:

• google-adk: main branch (post-1.26.0)
• Python: 3.13

Model Information:

• Are you using LiteLLM: No
• Which model is being used: claude sonnet 4.6 (Other models as well)

Additional Context:
Add any other context about the problem here.

Suggested fix

Add a fallback that serializes arbitrary response dicts:

# In part_to_message_block(), after existing content/result checks:
else:
    content = json.dumps(response_data)

This ensures any FunctionResponse dict is passed to the model, regardless of key names.

How often has this issue occurred?:

• Always (100%)