* test: add unit tests for langfuse context and noop observer
Add tests for observation context put/get round-trip, nested context
override behavior, and noop observer: trace ID generation, parent
inheritance, explicit override, Shutdown, ForceFlush, and enqueue.
* test: add edge case tests for noop observer branch coverage
Add two missing branch tests: explicit ObservationID with no parent
context (verifies new trace generation), and explicit TraceID with
parent (verifies observation ID is NOT inherited when trace is explicit).
Add comprehensive unit tests for all utility functions in the langfuse
helpers package: mergeMaps, ObservationLevel, GenerationUsageUnit,
GenerationUsage, ModelParameters, GetLangchainModelParameters, trace/span
ID generation, time helpers, and pointer helpers.