feat(ai): record expected MCP failure messages on the dispatcher span - #1961

🟢 Confidence 10/10 · safe to merge
A four-line span annotation with both presence and the 500-character cap pinned by tests in dispatcher.test.ts; no behavior outside telemetry changes.
_{no findings · tests covered · risk low}

Adds maple.mcp.error.message — a 500-character prefix of the failure text — to the dispatcher span so an expected MCP 4xx can be diagnosed from traces without joining logs. Contained and tested; safe to merge.

• recordExpectedMcpFailure annotates maple.mcp.error.message, capped at MAX_SPAN_MESSAGE_CHARS
• dispatcher.test.ts pins the cap and that a short message arrives whole

_{7d6b523 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}