Track removing Brotli from pylance's DataFusion FFI dependency graph

pylance currently keeps the Python DataFusion FFI integration exposed through FFILanceTableProvider. Keeping that API means pylance still pulls in Brotli through DataFusion's FFI dependency graph:

pylance
  -> datafusion-ffi
     -> datafusion-proto
        -> default = ["parquet"]
        -> datafusion-datasource-parquet
        -> parquet
        -> brotli

Downstream cannot disable this by adding datafusion-proto = { default-features = false } in pylance, because Cargo features are additive. The datafusion-ffi -> datafusion-proto dependency edge enables datafusion-proto/default, so pylance cannot turn it off from its own manifest.

This also exposes us to the current Brotli allocator resolution issue. A fresh Python lockfile resolve can select alloc-stdlib 0.2.3, which depends on alloc-no-stdlib >=2.0.4, <4.0.0, while brotli 8.0.3 also depends on alloc-no-stdlib 2.0. Cargo may resolve both alloc-no-stdlib 2.0.4 and 3.0.0, causing Brotli to fail to compile with allocator trait mismatches.

For now we should keep the Python FFI API stable and pin the Python-side allocator dependency resolution to the known working combination. Longer term, we should remove this pin once upstream DataFusion makes datafusion-ffi depend on datafusion-proto with default-features = false, or otherwise exposes a feature path that lets downstream users avoid Parquet/Brotli when they only need FFI table provider support.

Acceptance criteria:

• pylance keeps exporting FFILanceTableProvider.
• Python CI continues to pass with --locked.
• The temporary allocator pin is removed once the upstream DataFusion dependency edge no longer forces datafusion-proto/default.
• cargo tree --manifest-path python/Cargo.toml -i brotli no longer finds Brotli after the upstream dependency graph is fixed.

Start with python/Cargo.toml and inspect the datafusion-ffi dependency path, then run cargo tree --manifest-path python/Cargo.toml -i brotli. Check the Python lockfile and Python CI with --locked; done means FFILanceTableProvider remains exported, CI passes, and Brotli is absent once upstream removes the forcing dependency edge.