Feat/test new mu alt

[tomkosm]

Summary by cubic

Introduces an experimental MU v2 PDF parsing path sampled by an env-configured percentage, running in parallel to MU v1. Adds detailed telemetry for both paths without changing the primary scrape result.

• New Features
  • Gate and rollout via PDF_MU_V2_EXPERIMENT, PDF_MU_V2_BASE_URL, and PDF_MU_V2_EXPERIMENT_PERCENT.
  • Fire-and-forget MU v2 request with robustFetch; logs duration, tokens in/out, and page count; respects abort/mocks.
  • MU v1 instrumentation: start time, warnings on failure/no result, and completion logs with duration and URL.

[cubic-dev-ai]

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