fix(ai): tolerate fenced provider JSON in suggestions, harden prompt

[pratikbodkhe]

Problem

POST /api/meetings/[meetingId]/suggestions returned 502 "AI provider returned invalid suggestions" in production. RCA: the provider (google/gemini-3.1-flash-lite, prior minimax-m3) wraps JSON in markdown fences despite response_format: json_object. The suggestions route carried its own divergent getTextFromOpenRouter that skipped fence stripping, so JSON.parse threw. The ask and enhance-notes routes were already hardened (commit b01e7f4); suggestions was the only one missed.

Changes

• fix: suggestions route reuses the shared fence-aware getTextFromOpenRouter from @/lib/ai/ask-series-answer (deletes the local non-stripping copy).
• product: hardened the suggestions prompt with output-contract guardrails (JSON-only, no fences, {"suggestions": []} when nothing qualifies, verbatim source_excerpt, never invent owners/dates, confidence calibration, category definitions). Defense-in-depth alongside the parse-side fix.
• cleanup: deduped enhance-notes onto the same shared helper (removed a weaker forked parser + redundant double fence-strip; compacted prompt context JSON).
• tests: AI notes contract verifier now asserts both routes reuse the fence-aware helper and that the suggestions prompt carries its guardrails (red before, green after).

Verification

• pnpm build (full app type-check): pass
• verify-ai-notes-contract, verify-query-contracts, verify-runtime-config: pass
• eslint on changed files: clean

Follow-up (separate PR)

Extract a shared src/lib/ai/openrouter.ts (callOpenRouter + getOpenRouterApiKey) to collapse the 3-way getOpenRouterData/URL/apiKey duplication across all three AI routes, and tighten the enhance-notes select("*") over-fetch with a typed query.