feat(DATAGO-122217): Add ssl_verify config option for A2A proxy

[gregmeldrum]

What is the purpose of this change?

Add a boolean configuration option (ssl_verify) to disable SSL certificate verification for individual A2A proxied agents. This allows connecting to remote A2A agents using self-signed certificates in development/testing environments.

How was this change implemented?

• config.py: Added ssl_verify field to A2AProxiedAgentConfig with default value True
• component.py: Applied ssl_verify setting to httpx clients for both agent card fetching and task invocation
• proxies.md: Added documentation section on SSL verification with security warnings
• a2a_proxy_example.yaml: Added commented example showing ssl_verify: false usage

Key Design Decisions

• Default is True (verify certificates) to maintain security by default
• Setting applies to both agent card fetching and task invocations for consistency
• Per-agent configuration allows mixed environments (some verified, some not)

How was this change tested?

• Unit tests: Added 7 new tests in tests/unit/agent/proxies/a2a/test_config.py
• Ruff linting: All checks pass
• Manual testing: Not yet tested with actual self-signed certificate

Is there anything the reviewers should focus on/be aware of?

• Security documentation warns users about the risks of disabling SSL verification
• The setting only affects HTTPS connections; HTTP connections are unaffected