fix: read context records through MemWAL scanner

[beinan]

Summary

• Route ContextStore::list through Lance's MemWAL LSM scanner so flushed WAL generations are visible to reads.
• Compute semantic search over LSM-visible records and restore the MemWAL key-path assertions.
• Keep list pagination semantics explicit after collection instead of relying on an offset-only scanner limit.

Testing

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• uv run --extra tests pytest python/tests/ -v
• uv run --extra dev ruff format --check python/
• uv run --extra dev ruff check python/
• uv run --extra dev pyright

Notes

• The repo helper script .codex/skills/ci-pr-helper/scripts/run_ci_checks.sh currently references rust/lance-context/Cargo.toml, which does not exist in this workspace, so I ran the current CI-equivalent commands above.
• uv run --extra tests pytest without the CI path filter still has existing failures in tests/test_compaction.py and tests/test_persistence.py; those tests directly inspect base Lance fragments/rows or moto S3 behavior outside the MemWAL-aware read path.