feat(tracing): support multiple Langfuse clients

[kxzk]

Summary

• Move tracing creation onto client-owned providers while keeping module-level APIs as the singleton facade.
• Keep observation follow-up operations attached to their owning client for child observations, masking, trace URLs, scoring, flush, and shutdown.
• Document multi-client usage and add an SDK-plus-CLI validation workflow.

Verification

• bundle exec rspec
• bundle exec rubocop
• SDK validation emitted a real trace with a root span, child generation, and trace score; CLI readback confirmed the trace and score. The observations list endpoint was unavailable on the configured self-hosted instance, so trace readback with observations included was used for that check.

[linear-code]

AAI-130