fix(server/a2a): structured error parity with MCP — emit field/details/retry_after, catch decisioning AdcpError

[bokelley]

Closes #530

Summary

The A2A transport had two parity gaps vs. the MCP structured-error path shipped in #525:

1. execute() only caught adcp.exceptions.ADCPError — decisioning-layer adcp.decisioning.types.AdcpError (raised by DecisioningPlatform adopters) fell into the generic except Exception path and emitted a plain "Skill execution failed" text message with no structured envelope.

2. _send_adcp_error only emitted code/message/recovery/suggestion — the field, details, and retry_after fields were dropped even when the raised error carried them, breaking wire-conformance for A2A storyboard structured-error assertions.

This PR:

• Adds a except Exception branch in execute() that lazy-imports AdcpError from adcp.decisioning.types (matching the pattern in translate.py) and routes it through _send_adcp_error.
• Refactors _send_adcp_error to delegate to _extract_structured_fields from translate.py (introduced in feat(server): MCP error responses populate structuredContent.adcp_error (closes #509) #525), producing the full {code, message, recovery, field, suggestion, details, retry_after} envelope — and always emitting recovery unconditionally to match the MCP path (transport-errors.mdx §A2A Binding).
• Removes now-unused ADCPTaskError and STANDARD_ERROR_CODES imports.

What tested

• pytest tests/test_a2a_server.py — 47 passed (7 new tests added)
• pytest tests/test_server_idempotency.py — 45 passed (existing A2A conflict test still green)
• pytest tests/ (non-integration, excluding 3 pre-existing missing-dep failures) — 3302 passed, 18 skipped
• ruff check src/adcp/server/a2a_server.py — clean
• mypy src/adcp/server/a2a_server.py — no new errors (pre-existing a2a/protobuf stub gaps unchanged)

Pre-PR review

• code-reviewer: approved — no blockers. Nits noted: (1) lazy _DecAdcpError import inside except is redundant with _extract_structured_fields's own lazy import (harmless, Python module cache); (2) exc: Any on _send_adcp_error exposes a potential silent TypeError for future callers (internal callers are guarded); (3) prefer DecisioningAdcpError over _DecAdcpError for local var naming. All nits deferred.
• ad-tech-protocol-expert: approved — one blocker found and fixed before push: recovery was guarded by if recovery: (could omit on falsy string) vs. MCP path which emits it unconditionally. Fixed in commit c4f01a2 by moving "recovery": recovery into the base dict literal. Wire shape now fully matches transport-errors.mdx §A2A Binding.

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_019DMGxsA43mtT7QTRiNipBM

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Generated by Claude Code

[bokelley]

Superseded by #536 (feat(server/a2a): structured error parity with MCP — emit field/details/retry_after, catch decisioning AdcpError), already merged to main.

[bokelley]

Acknowledged — #536 supersedes this PR and is already merged to main. No further action needed here.

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Triaged by Claude Code. Session: https://claude.ai/code/session_0189ZzC2Brh3uWR3b8fq1Xiv

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Generated by Claude Code