* test: move offline router, latency-routing, retry and exception tests from local_testing into tests/unit
Moves 29 offline nodes into the tests/unit files that mirror the code they exercise.
Hosted-provider calls are mocked at the HTTP boundary with respx, and sleeps that only
built a start/end gap are replaced by explicit timestamps. Legacy files left empty are deleted.
* ci: record the live prompt caching cases left in tests/local_testing as unrun
Moving the router prompt caching test to tests/unit leaves only live Anthropic and Vertex cases in tests/local_testing/test_anthropic_prompt_caching.py, which every CircleCI -k already deselects
* test: freeze the clock in the moved lowest-latency routing tests
The latency logger buckets usage by the current minute, so a run that crosses a minute boundary could grow the cache and fail the memory check. The moved tests now use fixed timestamps and a frozen clock in the logger module
* ci: describe the mixed prompt caching file accurately in the coverage allowlist
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