Split docs extension acceptance modules

[yvette-carlisle]

Summary

• Split docs extension acceptance helpers into context, outbox, worker, qdrant, filter fixture, and assertion modules.
• Split docs extension lifecycle coverage into end-to-end, delete, source library, filter, and write policy modules.
• Split docs extension search filter coverage into timestamp, sparse, domain/repo, and recency modules.

Validation

• RUSTC=/run/current-system/sw/bin/rustc cargo make fmt-rust
• RUSTC=/run/current-system/sw/bin/rustc cargo make check (403 passed, 92 skipped)