fix: reject empty column alterations and unknown Python keys (#4417)

Fixes #4390

Lance accepts column alterations with every change field unset, so a
path-only Python call could commit a new local version without changing
the schema. The Python binding also silently discarded unknown keys such
as `nulable`.

Validate every alteration in the shared Rust `Table` API before
dispatching to either backend, and reject unknown dictionary keys in the
Python binding while preserving the legacy `name` alias. Invalid batches
leave local schema and version unchanged and send no remote request.

Validation:

- Reproduced both reported calls on the original extension: versions
advanced from 1 to 2 to 3 with an unchanged schema; all 13 new
invalid-input Python cases failed before the fix.
- Rebuilt the extension with `uv run --extra tests --extra dev maturin
develop --extras tests,dev`; all 18 focused Python cases pass across
sync/async and local/mocked remote tables.
- `cargo test --quiet --features remote -p lancedb --lib alter_column`:
all eight tests pass, including both new regressions.
- `cargo clippy --quiet --features remote --tests --examples` passes
across the workspace.
- `cargo fmt --all`, `ruff check .`, and `ruff format .` pass.
- `typos` 1.26.0 passes across the repository; intentional misspellings
in regression fixtures use the existing line-specific spelling
exemptions.

<!-- lance-gatekeeper-fix:v1 agent=36a547cd11108898893456042e38628b
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/src/table.rs

@@ -1897,6 +1897,17 @@ impl Table {
         let alterations = alterations
             .iter()
             .map(|alteration| {
+                for key in alteration.keys().iter() {
+                    let key = key.extract::<String>()?;
+                    if !matches!(
+                        key.as_str(),
+                        "path" | "rename" | "name" | "nullable" | "data_type"
+                    ) {
+                        return Err(PyValueError::new_err(format!(
+                            "Unknown column alteration key '{key}'"
+                        )));
+                    }
+                }
                 let path = alteration
                     .get_item("path")?
                     .ok_or_else(|| PyValueError::new_err("Missing path"))?