refactor(observability): collapse duplicates after beta.3

[arapov]

Summary

Post-beta.3 cleanup pass on the observability code. No behaviour change — same events, same fields, same gating. Three targeted simplifications that reduce duplication and make the contracts clearer.

Changes

1. Extract emitToolCall helper (src/server/register-tool.ts)

Three near-identical logEvent("tool.call", ...) blocks (registerTool success branch, registerTool error branch, registerToolTask IIFE) collapse to one-line calls. Single place to change the event shape.

2. Move tool.chain emission into withRequestContext (src/log.ts + src/http/app.ts)

The chain lifecycle now lives in the helper that creates the context. Callers can never forget to emit. src/http/app.ts's /mcp handler shrinks to a clean three-line wrapper:

await withRequestContext({ clientId }, async () => {
  await server.connect(transport);
  await transport.handleRequest(req, res, req.body);
});

The emission fires on scope exit even when fn throws (try/finally), so partial chains stay observable on error paths.

3. Replace inline if/else with dispatch table (src/log.ts)

The per-event ctx-mutation logic in logEvent becomes a declarative chainHandlers table. Adding a new event type that should feed the chain is now one row.

Test plan

• npm run typecheck clean
• npm run lint clean
• npm run format:check clean
• npm test — 460 / 460 (no test count change, refactor)
• npm run build succeeds — dist/index.js 144.74 KB, dist/http.js 170.83 KB
• One existing test (tool.chain aggregates the request's tools and capsule calls) updated to drop its now-redundant manual emission — withRequestContext does it automatically
• Privacy sweep clean

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