docs(tracing): add Langfuse tracing guide and fix OTLP endpoint example

[Ethan-Xingyue]

Which issue or RFC does this PR close?

No dedicated issue. This delivers the "Langfuse integration" item assigned in Discussion #1402 ("20260830 weekly",
observability and evaluation table) and in the 2026-08-16 meeting notes (docs/en/meetings/2026-08-16.md). It stays
within RFC 0046 (Observability Foundations): PowerContext keeps exporting standard OTLP, and no vendor-specific
configuration is added.

Rationale for this change

Operators asked whether PowerContext traces can be viewed in Langfuse, which adds cost accounting, a metrics API, and
metadata filtering on top of the span tree that the Phoenix guide already documents. The verification showed that the
existing OTLP export works unchanged: Langfuse ingests every PowerContext span through its OTLP endpoint when the
standard OpenTelemetry environment variables carry the Langfuse endpoint and Basic-auth header. The only missing piece
was documentation.

The .env.example OTLP endpoint example also carried a /v1/traces suffix that the OpenTelemetry HTTP exporter
appends again (_append_trace_path() in opentelemetry-exporter-otlp-proto-http 1.43.0), which would send spans to
/v1/traces/v1/traces.

What changes are included in this PR?

• New how-to guide docs/en/docs/how-to/trace-with-langfuse.md and its Chinese translation
  docs/zh/docs/how-to/trace-with-langfuse.md, structured like the Phoenix guide: start Langfuse, install the
  tracing-otlp extra, configure the exporter with OTEL_EXPORTER_OTLP_ENDPOINT / OTEL_EXPORTER_OTLP_HEADERS,
  trigger one flush, read the resulting observations (types, names, metadata, request-ID filtering, cost), what is
  not exported, stop Langfuse.
• One cross-reference sentence in the OpenTelemetry section of docs/en/docs/reference/configuration.md and
  docs/zh/docs/reference/configuration.md.
• .env.example: the OTEL_EXPORTER_OTLP_ENDPOINT example is now a base URL with a comment that the SDK appends
  /v1/traces.

No code, dependency, span name, or attribute changes.

Are there any user-facing changes?

Documentation only. The .env.example change corrects a commented-out example value; it does not change runtime
behavior.

How was this change tested?

• make check (lock consistency, prek hooks, ty) passed.
• make docs-test (oxlint plus the Next.js static export and its verification) passed on top of 74b961fb; both new
  pages are generated and listed in the how-to sidebar.
• uv run pytest tests/test_server_tracing.py tests/e2e/test_observability.py: 24 passed. The default test suite still
  needs no telemetry backend.
• Manual end-to-end verification against a local Langfuse 4.10.0 (Docker Compose) with PowerContext at f0f288ab:
  one Server run without a generation model and one with Pydantic AI's test model, driving HTTP (/v1/sources/content,
  /v1/memory/flush, /v1/memory/remember, /v1/memory/search, /v1/context/prepare) and MCP (list_memory_entries)
  requests. Exported observations were read back through GET /api/public/v2/observations: 67 observations across 19
  traces, all PowerContext spans ingested as SPAN, invoke_agent as AGENT, chat <model> as GENERATION with
  model name and token usage, FastMCP tool spans as TOOL; failed operations mapped to level=ERROR with
  error.type; attributes.powercontext.request.id filterable through the metadata filter. A grep of the exported
  JSON found no Memory or Source content, search queries, scope_id, authorization headers, or keys; generation
  input/output contain message shapes only.
• Not verified: a real provider model (TestModel was used), embedding spans, the Langfuse UI navigation labels (API
  only), and Langfuse Cloud.