chore: simplify langfuse module + multi-step coordinator

[iskakaushik]

Summary

Cleanup pass over the langfuse + multi-step coordinator code that just landed (#40, #42). No behaviour changes for the public API; simplifications and dependency hygiene.

Langfuse module

• One ISO-8601 helper (Trace::to_iso8601(time_point)) replaces three duplicated `gmtime_r`/`snprintf` blocks (the named static plus two inline lambdas in `finish_generation` / `build_trace_event`).
• Capture timestamp once per span event instead of two separate clock reads.
• Magic-string constants in the anonymous namespace for the Langfuse event type discriminators (`trace-create` / `span-create` / `span-update` / `generation-create`) and for level / unit strings.
• Shared `make_span_create` + `wrap_event` helpers so the orphan-span fallback in `record_tool_call_finish` reuses them instead of duplicating the body-construction.
• Drop over-engineered `ParsedHost`: `httplib::Client`'s URL constructor handles scheme/host/port already; we only need a small `extract_base_path` for Langfuse instances served under a sub-path.
• Cache `httplib::Client` + Basic-auth headers on `Tracer` (lazily constructed under `mu_`) so repeat `send_batch` calls reuse the TLS session.
• Replace 6-arg `start_trace` with `start_trace(name, TraceOptions{})` to match the rest of the SDK's struct-of-options style.
• Strict by default: `Config::error_policy = ErrorPolicy::kStrict` (was `best_effort = true`) so misconfigurations surface at integration time instead of being silently swallowed.
• Reject post-end mutations: `set_` / `instrument` / `finish_generation` / `record_` bail out once `Trace::end()` has fired.
• Replace hand-rolled UUID v4 with stduuid (header-only, MIT, vendored under `third_party/stduuid-header-only/`). Wired into `ai-sdk-cpp-langfuse` via a PRIVATE link.

Multi-step coordinator

• Avoid re-copying `initial_messages` every loop iteration. `step_messages` is grown in place: `erase` back to the immutable prefix + `insert` the running `response_messages` accumulator, instead of `step_options = initial_options; step_options.messages = initial_messages; step_options.messages.insert(...)` per step.
• Remove dead `create_next_step_options` stub from the public header + impl (not part of any external ABI; was only kept as scaffolding during the refactor).
• Trim a narrating comment that referenced an external repo path.

Test plan

• `make ai_tests langfuse_tracing` clean
• Full `ctest` — 221/227 pass; the 6 failures are `ClickHouseIntegrationTest.*` cases that need a local ClickHouse server (unchanged from `main`)
• End-to-end `langfuse_tracing_debug` against `us.cloud.langfuse.com` — trace ingested with 1 generation + 2 tool spans nested correctly under it
• Multi-step duplicate-execution suite — 4/4 pass for both providers