Check A2A_USER_INPUT environment variable before prompting for input

[Copilot]

Addresses feedback on #44615 to align sample_agent_to_agent.py with the pattern used in fabric and bing custom search samples.

Changes

• Updated sample_agent_to_agent.py line 70 to check A2A_USER_INPUT environment variable before prompting for user input
• Implementation now matches the documented behavior (line 29) and pattern used in sample_agent_fabric.py and sample_agent_bing_custom_search.py

# Before
user_input = input("Enter your question (e.g., 'What can the secondary agent do?'): \n")

# After
user_input = os.environ.get("A2A_USER_INPUT") or input("Enter your question (e.g., 'What can the secondary agent do?'): \n")

This allows users to set A2A_USER_INPUT to skip interactive prompts, consistent with other agent tool samples.

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[Copilot]

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