chore: clean up redundant inline docs in a2a module

[greysonlalonde]

Note

Low Risk
Low risk because this PR only removes redundant inline comments/doc examples without changing any runtime behavior or APIs.

Overview
Cleans up redundant inline documentation in the A2A module by removing unnecessary comments from the ERROR_MESSAGES mapping, the A2AExtension protocol example, and extract_a2a_agent_ids_from_config config filtering logic.

No functional behavior changes; this is a readability-only documentation tidy-up.

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[iris-clawd]

Clean comment removal — all 6 deleted lines are redundant inline comments that restate what the code already says. No behavior changes. LGTM.