fix(runtime): support Langfuse v3/v4 trace endpoints

[wutongyuonce]

Summary

• Fall back from GET /api/public/traces to v2 observations for Langfuse v4 events_only deployments.
• Fall back from v2 observations to legacy GET /api/public/observations for Langfuse v3 self-hosted deployments.
• Aggregate observation rows into trace summaries and preserve deployment-specific 404 details.
• Add regression tests covering both fallbacks and authentication failures.

Closes #146

Validation

• bun test packages/runtime/tests — 106 passed
• bun run lint — passed
• bun run typecheck — passed
• bun test — 773 passed, 1 unrelated failure because the local Python environment lacks jinja2 (templates.test.ts)

[wutongyuonce]

@MagicCube could you take a maintainer review when you have a moment?

This addresses #146: Langfuse 3.110 is missing /api/public/v2/observations, and Langfuse v4 events_only is missing GET /api/public/traces. The client now falls back traces → v2 observations (aggregated to trace summaries) and v2 observations → legacy /api/public/observations, and keeps the deployment-specific 404 body in the error.

Locally: bun test packages/runtime/tests 106 passed; bun run lint / typecheck passed.