fix(a2a): redact error event messages

[XnLemon]

Objective

Redact model-visible A2A agent error event messages for Phase2 redaction closure.

Completed Changes

• Route A2A sendErrorEvent ResponseError.Message through the shared platform redactor.
• Preserve structured ResponseError fields such as type, code, and param.
• Add regression coverage for Authorization, api_key, and Cookie leakage strings in A2A error events.

Validation

• go test ./agent/a2aagent
• go vet ./agent/a2aagent
• git diff --check
• Subagent open-code-review: 0 High / 0 Medium / 0 Low

Known Risks / Limitations

• This increment is scoped to A2A sendErrorEvent; remaining Phase2 model-visible error or metadata surfaces should be audited in follow-up increments.

Follow-up Work

• Continue Phase2 closure by scanning remaining model-visible error surfaces outside agent-specific sendErrorEvent paths.

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 204bc84a-f0c3-4449-91a7-4d683a913344

📥 Commits

Reviewing files that changed from the base of the PR and between 361e2a4 and 064dbfb.

📒 Files selected for processing (2)

• agent/a2aagent/a2a_agent.go
• agent/a2aagent/a2a_agent_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/phase2-a2a-error-event-redaction

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