chore(diag): expose hasDatabaseUrl + instanceId in _proxy_debug (#318)

The Phase 3 rate limit is partially working in production —
12 streaming requests resulted in 7 DB inserts but no 429s.
Theory: some Vercel function instances cold-start without
DATABASE_URL populated and silently fail-open for their lifetime.

This commit adds three fields to the /api/_proxy_debug response so
we can confirm:
- hasDatabaseUrl: whether the env var is visible at runtime
- rateLimitConfigured: whether the proxy was wired with the hook
- instanceId: a per-instance random id, so multiple curls can show
  whether they hit the same instance or different ones

Once verified, this commit can stay (cheap) or be reverted as
preferred — the cost is 4 lines in the debug response.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: blove

scripts/langgraph-proxy.ts

@@ -105,6 +105,13 @@ export function createProxyHandler(config: ProxyConfig = {}): (req: VercelReques
         query: req.query,
         hasApiKey: !!apiKey,
         apiKeyPrefix: apiKey.substring(0, 10),
+        hasDatabaseUrl: !!process.env['DATABASE_URL'],
+        rateLimitConfigured: !!config.checkRateLimit,
+        instanceId: (() => {
+          const g = globalThis as { __instanceId?: string };
+          if (!g.__instanceId) g.__instanceId = Math.random().toString(36).slice(2, 10);
+          return g.__instanceId;
+        })(),
       });
       return;
     }