chore: update lance dependency to v15.0.0-beta.6 (#4742)

Updates Rust and Java Lance dependencies to
[v15.0.0-beta.6](https://github.com/lance-format/lance/releases/tag/v15.0.0-beta.6),
pins Python tests to published pylance 15.0.0b6, and refreshes both
lockfiles.

Aligns Arrow 59, DataFusion 55, and PyO3 0.29; adapts execution plans
and MemWAL directory handling, updates the ABI metadata test, and
removes the obsolete Lance Arrow patch now fixed upstream.

Validation: workspace clippy with all features/tests and warnings
denied, cargo fmt, Python/CI Ruff checks, uv lock check, 16 CI-script
tests, and targeted Python regressions (54 passed, 28 skipped);
repository-wide Ruff retains existing documentation-example failures,
and the upstream macOS wheel build failed while Linux and Windows wheels
are published.

---------

Co-authored-by: Daniel Rammer <hamersaw@protonmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/rust.yml

@@ -381,7 +381,7 @@ jobs:
     runs-on: ubuntu-24.04
     strategy:
       matrix:
-        msrv: ["1.94.0"] # This should match up with rust-version in Cargo.toml
+        msrv: ["1.95.0"] # This should match up with rust-version in Cargo.toml
     env:
       # Need up-to-date compilers for kernels
       CC: clang-18