cartesian: `computation_src` cache in `_make_extension_sources` never hits (`backend_data` returns a copy)

[egparedes]

Note

This issue was created by a coding agent (Claude) acting on behalf of @egparedes.

cc @FlorianDeconinck @romanc. Found while working on #2927.

Description

BaseGTBackend._make_extension_sources in src/gt4py/cartesian/backend/gtc_common.py tries to cache the generated C++ sources in the builder's backend data, but the cache is never actually stored, so it never hits:

def _make_extension_sources(self) -> dict[str, dict[str, str]]:
    """Generate the source for the stencil independently from use case."""
    if "computation_src" in self.builder.backend_data:          # L265
        return self.builder.backend_data["computation_src"]     # L266
    ...
    self.builder.backend_data["computation_src"] = gt_pyext_sources   # L280
    return gt_pyext_sources

StencilBuilder.backend_data returns a copy of the stored dict (src/gt4py/cartesian/stencil_builder.py, L219-220):

@property
def backend_data(self) -> dict[str, Any]:
    return self._build_data.get("backend_data", {}).copy()

so the assignment at L280 writes into a temporary copy that is immediately discarded, and the lookup at L265 never finds computation_src. The supported way to store backend data is self.builder.with_backend_data({...}), as done e.g. in numpy_backend.py (L82-84) and backend/base.py (L310).

Impact

Effectively none today: the only caller of _make_extension_sources is make_extension (gtc_common.py, L230), which runs once per build, so even a working cache would never be read. The code is therefore dead, but misleading.

Possible fixes

1. Remove the cache (L265-266 and L280). Simplest, since nothing would read it.
2. Make it work by storing it with self.builder.with_backend_data({"computation_src": gt_pyext_sources}). Only worth it if _make_extension_sources is expected to be called more than once per build.

Option 1 seems preferable unless there are plans to reuse the generated sources.