feat(langfuse): add Langfuse tracing module for ai::Client agent calls

[iskakaushik]

Summary

New optional `ai::langfuse` component that wraps `ai::Client::generate_text` calls and emits Langfuse traces for the LLM round-trip and each tool invocation.

Trace shape

• 1 trace per logical operation (caller sets input / output / metadata / tags)
• 1 `generation` observation (model, modelParameters, input messages, output text, aggregated usage, finish_reason / step count in metadata)
• 1 `span` per tool call, parented to the generation, with tool args as input and result (or error) as output

Public API (`include/ai/langfuse.h`)
```cpp
ai::langfuse::Tracer tracer({.host = "https://us.cloud.langfuse.com\",
.public_key = ..., .secret_key = ...});
auto trace = tracer.start_trace("sql-generation");
trace->set_input(prompt);
auto result = ai::langfuse::generate_text(client, std::move(options), *trace);
trace->set_output(result.text);
trace->end(); // synchronous batched flush
```

The module hooks into the existing `on_tool_call_start` / `on_tool_call_finish` callbacks on `GenerateOptions` and chains any user-installed callbacks. `Trace::instrument` mints the parent generation observation; `Trace::finish_generation` (called by the convenience wrapper) finalizes it after `generate_text` returns. `Trace::end` POSTs the batch to `/api/public/ingestion` via the already-vendored httplib + Basic auth.

Layout

• `include/ai/langfuse.h` – public surface (Config, Tracer, Trace, free `generate_text`)
• `src/langfuse/tracer.cpp` – HTTP / batching / event-shape implementation
• `examples/langfuse_tracing.cpp` – end-to-end demo using OpenAI with two tools across multiple steps
• `.env.local.example` – credentials template (`.env.local` itself is gitignored)
• `CMakeLists.txt` – new `ai-sdk-cpp-langfuse` target (alias `ai::langfuse`), `AI_SDK_HAS_LANGFUSE=1`, in `ai::sdk` umbrella and install/export set

Scope cuts (intentional, MVP)

• Synchronous flush on `Trace::end()`; no background thread yet.
• `on_step_finish` is not used to emit per-step generation events; intermediate step text/usage roll up into the single parent generation. Easy to add later.

Test plan

• `make ai-sdk-cpp-langfuse` clean
• `make langfuse_tracing` example links and runs
• End-to-end: trace ingested at `us.cloud.langfuse.com` (HTTP 207 with all events accepted), retrieved via `GET /api/public/traces/`; structure verified — 1 generation + N tool spans with proper input/output and parent-child links
• No effect on existing `ai-sdk-cpp-core` / `-openai` / `-anthropic` builds