feat(python): expose Function GPU classes and placement (#4791)

## Summary

- Add sync/async `DBConnection.list_gpu_classes()` returning typed
`GpuClass` records for `GET /v1/gpu/classes`. Records include the class
name, Kubernetes resource, GPU model and memory, per-worker GPU maximum,
and worker CPU/memory/ephemeral-storage limits.
- Add `udf(gpu_class=...)` for immutable Function-level GPU placement.
It requires `gpu=True`; Function versions expose their registered
`gpu_class`.
- Add `gpu_class` and `gpu_count` to `refresh_column` /
`refresh_column_async` and Rust `RefreshColumnOptions`. Both are sent as
top-level `backfill_column` fields and validated before any request.
`gpu_count` is the aggregate GPU budget across Function workers.
- Validate GPU class names with the server lowercase 63-character
DNS-label rule.
- Document the discovery-to-registration flow, configured hardware
versus free capacity, System Usage permission, and per-refresh GPU
placement in the API and web guides.

Older servers without `/v1/gpu/classes` produce a clear
`NotImplementedError` instead of a bare 404.

Part of ENT-3470. The client follows the in-flight Sophon contract on
`xuanwo/ent-3470-fix` for GPU class discovery and backfill placement
options.

## Breaking change

`PythonRuntimeSpec::PythonV2` adds `gpu_class`, and
`RefreshColumnOptions` adds `gpu_class` and `gpu_count`. Rust callers
constructing either value with all fields explicitly must supply the new
fields.

## Validation

At `7685c968`: Rust GPU tests (5 passed), Rust/PyO3 clippy, Python
extension build, pyright, ruff, and Python Function/runtime, discovery,
refresh placement, and refresh regressions (147 passed, 6 skipped).

At `b22428fd`: Rust GPU tests (5 passed), rustfmt, ruff, pyright,
Function slice 2 (85 passed), and remote GPU/refresh tests (20 passed).

The final commit only updates documentation and Rust doc comments.

python/src/lib.rs

@@ -65,6 +65,7 @@ pub fn _lancedb(py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
         .write_style("LANCEDB_LOG_STYLE");
     env_logger::init_from_env(env);
     m.add_class::<Connection>()?;
+    m.add_class::<connection::GpuClass>()?;
     m.add_class::<catalog::Catalog>()?;
     m.add_class::<catalog::DatabaseNames>()?;
     m.add_class::<listing::NameListing>()?;