Add sanitized fixture record and replay

[clifton]

Summary

• add a versioned Fixture format and an LLMClient-compatible FixtureRecorder<C> for non-streaming calls
• require a caller-supplied sanitizer before any request or response is retained, structurally redact common credential fields, and never persist inline media bytes
• add strict ordered ReplayClient matching for operation, sanitized prompt, target schema, and media metadata
• preserve token usage, complete extraction reports, typed errors, HTTP status, and request IDs across record/replay
• add a runnable key-free example and a checked-in real-world Apple 2023 10-K fixture

Behavior

Replay mismatches report only the differing field and leave the interaction unconsumed. ReplayClient::assert_finished() catches tests that skipped recorded calls. Fixture JSON is pretty, deterministic, version-checked, and available through both file and string APIs.

The mock feature remains dependency-light and owns the fixture API; provider HTTP dependencies are not required for replay.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --target-dir tmp/pr4-all-target -- -D warnings
• cargo check --no-default-features --features derive,mock --target-dir tmp/pr4-min-target
• all-feature library suite: 312 passed
• fixture integration suite: 2 passed, including canonical real-world replay and schema drift
• deterministic all-feature integration matrix (credentialed live-provider modules skipped)
• doctests: 84 passed
• commit-hook default-feature unit suite: 219 passed