feat(testing): add Langfuse::Testing module for in-memory span capture

[Alan-Marx]

Summary

We recently added the gem to our application and have set up an in-memory span exporter in our test suite so that we can test our observed code without needing to mock Langfuse network requests or stub Langfuse methods. We think this approach provides a pretty good DX and gets us pretty close to what is happening in production so thought we would offer it up to you guys for review.

• Adds require "langfuse/testing" + include Langfuse::Testing as a supported public API for asserting against emitted spans in tests without real network calls or private ivar access
• Introduces OtelSetup.test_mode flag that causes build_exporter to return a fresh InMemorySpanExporter on each provider build, surviving shutdown/reset! cycles transparently
• Provides two instance methods: emitted_langfuse_spans (flush + return recorded spans) and reset_langfuse (flush + clear buffer between tests); see docs/TESTING.md for RSpec and Minitest integration examples

Test plan

• bundle exec rspec spec/langfuse/testing_spec.rb — all examples pass
• bundle exec rspec — full suite passes (1347 examples, 0 failures)
• bundle exec rubocop — no offenses
• In an app: require "langfuse/testing", include Langfuse::Testing, call Langfuse.observe, assert on emitted_langfuse_spans — spans captured without network calls
• Confirm reset_langfuse clears spans between tests and emitted_langfuse_spans returns [] on a clean slate
• [ x Confirm test_mode survives Langfuse.reset! (no OTLP exporter used after reset)

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Note

Medium Risk
Changes the tracer provider initialization path and exporter selection, but production behavior stays on OTLP unless langfuse/testing is required; mis-loading that file in a non-test process would silently drop real exports.

Overview
Adds a test-only entry point (require "langfuse/testing") so apps can run the real Langfuse tracing pipeline and assert on exported spans without OTLP network calls or stubbing internal APIs.

Langfuse::Testing turns on OtelSetup.test_mode at load time and exposes emitted_langfuse_spans (flush + read finished spans) and reset_langfuse (flush + clear the in-memory buffer between examples). docs/TESTING.md documents RSpec/Minitest setup patterns.

OtelSetup now tracks the active span_exporter, wires the exporter through publish_provider (including rollback/shutdown cleanup), and build_exporter returns InMemorySpanExporter when test_mode is set—so in-memory export survives shutdown / reset! and each provider rebuild gets a fresh buffer. Specs cover publish-race behavior for span_exporter and the test-mode exporter path.

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