feat(examples): add Mastra TypeScript agent example + E2E test

[Rushit]

Goal

Add a Mastra TypeScript agent example with E2E span validation against zradar.

Skeleton

examples/mastra/ on docs_opt (TypeScript only):

examples/mastra/
  agent.ts              ← Mastra Agent with get_forecast tool
  package.json
  README.md
  tests/expected_spans.json

What "done" looks like

• just example mastra runs without error (mock when no API key)
• just example-test mastra validates spans in zradar
• Works with OPENAI_API_KEY set

How to test

just dev
just example-test mastra

[Rushit]

Path update: Example is now at examples/mastra/typescript/example.ts.