fix: coerce callable AgentInfo.instruction for app-info - #6913

Summary

• GET /apps/{app_name}/app-info built AgentInfo with instruction=current_agent.instruction. That field is typed str, but LlmAgent.instruction is str | InstructionProvider. A callable raised an unhandled pydantic.ValidationError (500).
• Coerce callables to a readable placeholder (<InstructionProvider: {name}>) in a before validator. Do not resolve the provider: it may be async and may need session state.
• String instructions are unchanged. Class-based providers (no __name__) use the type name.

Fixes #6909

Test plan

• Reproduced: get_agents_dict on an agent with instruction=dynamic_instruction raises ValidationError: Input should be a valid string (input_type=function) at agent_info.py _traverse / AgentInfo(...).
• After the fix, uv run pytest tests/unittests/utils/test_agent_info.py — 13 passed
  • named function → <InstructionProvider: dynamic_instruction> (provider is not invoked)
  • async provider + callable on a sub-agent
  • callable instance → <InstructionProvider: Persona>
  • plain string instruction still round-trips
• CLA check

@itaieban this is done — app-info now serializes callable instructions instead of 500ing.

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