Use JSON serialization for message arrays in telemetry tags

[Copilot]

Comma-separated concatenation of messages causes parsing ambiguity when individual messages contain commas.

Changes

• Replace ",".join(messages) with json.dumps(messages) in record_input_messages() and record_output_messages()
• Add import json to invoke_agent_scope.py

Example

# Before: ambiguous when messages contain commas
messages = ["Hello, world", "Test, message"]
tag_value = ",".join(messages)  # "Hello, world,Test, message" - cannot parse back correctly

# After: unambiguous serialization
tag_value = json.dumps(messages)  # '["Hello, world", "Test, message"]'

This makes telemetry data parseable using standard JSON instead of custom string splitting logic.

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