chore(closes OPEN-10363): clean up trace configuration

[gustavocidornelas]

Pull Request

Summary

Unifies tracer configuration around a new init() function with merge semantics and a single resolver. Today there are three overlapping ways to configure the tracer (env vars, configure(), per-decorator override) and they don't all agree — most notably, configure() silently resets unspecified options on each call, and the LangChain callback bypasses configure()'s state entirely.

This PR makes the three paths consistent and lays the foundation for a future init(auto_instrument=True) feature.

Changes

• Add init() as the new canonical entry point in openlayer.lib. Idempotent and merges with prior state — partial calls (e.g. init(api_key="X")) no longer reset unspecified options.
• Replace eleven _configured_* module globals with one _tracer_config dict + _UNSET sentinel.
• Add public resolvers resolve_api_key(), resolve_pipeline_id(), resolve_base_url() — single source of truth
  (_tracer_config > env var > default). Used by _get_client(), the @trace() decorator, and integrations.
• Fix LangChain callback bypass at langchain_callback.py:252 and :1310 — was reading OPENLAYER_INFERENCE_PIPELINE_ID directly via utils.get_env_variable, ignoring configure(inference_pipeline_id=...). Now routes through resolve_pipeline_id().
• Deprecate configure() — kept as a thin alias that emits DeprecationWarning (with stacklevel=2) plus a one-time INFO log, then delegates to init(). Slated for removal in v1.0.
• Add get_tracer_config() debug helper — returns the resolved config with API key redacted.
• Add one-time INFO log on first client build showing the resolved pipeline_id and base_url.
• Route attachment_uploader.py and the LangChain callback's background_publish_enabled check through _resolve() so no integration bypasses the resolver.
• Remove stale OPENLAYER_PROJECT_NAME reference in the @trace() docstring (it was never actually read anywhere).
• Rewrite tests/test_tracer_configuration.py with new merge-semantics, resolver, deprecation, and LangChain-bypass-fix tests.
  Update teardowns in test_tracing_core.py and test_offline_buffering.py to use tracer._tracer_config.clear().
• Rewrite examples/tracing/programmatic_configuration.py to recommend init() and demonstrate merge semantics; keep one labeled-deprecated configure() example.

Context

OPEN-10363: Clean up trace configuration

Testing

• Unit tests
• Manual testing

Monitoring

• No expected impact

Notes

• Behavior change for configure() callers: previously, configure(api_key="X") silently reset every other configured value. Under the new merge semantics, unspecified args are preserved. Pass an explicit None to clear a configured value. We are not aware of any callers relying on the old replace-on-omit behavior, but worth a callout in the changelog.
• All custom code stays under openlayer.lib/ per the Stainless constraint. The generated openlayer/__init__.py is untouched.
• Internal/edge-case env vars unchanged: OPENLAYER_DISABLE_PUBLISH and OPENLAYER_VERIFY_SSL remain read at import time and are intentionally not exposed via init().