Bug: thought metadata lost during A2A to GenAI part conversion #4242

Please make sure you read the contribution guide and file the issues in the right place.
Contribution guide.

Describe the bug

The convert_a2a_part_to_genai_part function ignores metadata when converting A2A parts back to GenAI parts. This causes the thought field to be lost during A2A communication between agents, breaking remote multi-agent scenarios where reasoning/thought content needs to be preserved.

To Reproduce

from google.genai import types as genai_types
from google.adk.a2a.converters.part_converter import (
    convert_genai_part_to_a2a_part,
    convert_a2a_part_to_genai_part,
)

# Create part with thought=True
original = genai_types.Part(text="thinking...", thought=True)

# Round trip conversion
a2a_part = convert_genai_part_to_a2a_part(original)
result = convert_a2a_part_to_genai_part(a2a_part)

# Bug: thought is lost
print(f"Original thought: {original.thought}")  # True
print(f"Result thought: {result.thought}")      # None (should be True)

Steps to reproduce:

1. Install google-adk
2. Run the code above
3. Observe that result.thought is None instead of True

Expected behavior

When a GenAI Part with thought=True is converted to A2A and back to GenAI, the thought field should be preserved. The round-trip should be lossless for metadata like thought.

Screenshots

N/A

Desktop (please complete the following information):

• OS: macOS / Linux / Windows (all affected)
• Python version: 3.11+
• ADK version: Current main branch

Model Information:

• Are you using LiteLLM: N/A (bug is in part conversion, not model interaction)
• Which model is being used: N/A

Additional context

This issue affects any multi-agent A2A scenario where:

• A sub-agent generates content with thought=True (e.g., reasoning steps)
• The content is sent to a parent agent via A2A protocol
• The parent agent receives the content but loses the thought marker

This makes it impossible for downstream consumers (like UIs) to distinguish thoughts from regular content.