Add diagnostic reasons to MCP attribution errors

[copyberry]

Add diagnostic reasons to MCP attribution errors

Why

The attribution_error status does not explain why MCP attribution is incomplete. Preserve a diagnostic reason so callers can distinguish history and recorder errors from request serialization failures.

What changed

• Add optional error_reason diagnostics to McpAttribution, retaining the first recorder error through checkpoints, compaction, resume, and fork.
• Identify missing or invalid checkpoints, conflicting sources, invalid sources, and poisoned recorder state. Use restored_error_unknown for older error checkpoints without a reason.
• Distinguish payload_too_large from serialization_failed in request metadata fallbacks.
• Tolerate missing, unknown, or malformed diagnostic reasons when reading history, without letting diagnostics determine attribution status.

Testing

Add coverage for reason preservation, recorder poisoning, backward-compatible checkpoint decoding, resume and cold-fork restoration, and request metadata serialization, including oversized payloads.

[github-actions]

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