Add mastra auto-instrumentation

[Stephen Belanger (Qard)]

Fixes #1890

Summary

• add Mastra auto-instrumentation configs and wire them into the hook, bundler plugin, webpack loader, and public exports
• add Mastra tracing channels and a runtime plugin for agent, tool, workflow run, and workflow step spans with current-span context binding
• add focused Mastra tests and an e2e scenario covering agent generate/stream, Tool.execute, workflow runs, and workflow-step tool nesting

Testing

• pnpm --dir js exec vitest run src/auto-instrumentations/configs/mastra.test.ts src/instrumentation/plugins/mastra-plugin.test.ts src/instrumentation/registry.test.ts
• pnpm run test:e2e -- mastra-instrumentation

Notes

• pnpm --dir js test -- mastra and pnpm --dir js test -- auto-instrumentations still hit unrelated existing network-dependent failures in src/framework.test.ts and src/logger-misc.test.ts
• pnpm run formatting still reports pre-existing unformatted fixture files under js/tests/auto-instrumentations/fixtures/test-files/

[Luca Forstner (lforst)]

Can we also add the mastra e2e test scenario to the ci summary?