Better defaults for native build backend cache keys (#15705)

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## Summary

This change extends the default initialized projects with scikit and
maturin build backends to ensure that the rust or C++ build gets invoked
when a source file changes.

## Test Plan

`cargo run init cppextension-test --build-backend=scikit` followed by
manual testing of the behaviour of the resulting project under `uv run
--with jupyter jupyter lab` and changes to the source file of the
extension.
Analogous for `cargo run init maturin-test --build-backend=maturin `.

## Relevant Issues

The question of why the python extension is not rebuilt on source
changes has been discussed in
https://github.com/astral-sh/uv/issues/15701#issuecomment-3258714942.

crates/uv/src/commands/project/init.rs

@@ -989,6 +989,9 @@ fn pyproject_build_system(package: &PackageName, build_backend: ProjectBuildBack
                 python-packages = ["{module_name}"]
                 python-source = "src"
 
+                [tool.uv]
+                cache-keys = [{{ file = "pyproject.toml" }}, {{ file = "src/**/*.rs" }}, {{ file = "Cargo.toml" }}, {{ file = "Cargo.lock" }}]
+
                 [build-system]
                 requires = ["maturin>=1.0,<2.0"]
                 build-backend = "maturin"
@@ -998,6 +1001,9 @@ fn pyproject_build_system(package: &PackageName, build_backend: ProjectBuildBack
                 minimum-version = "build-system.requires"
                 build-dir = "build/{wheel_tag}"
 
+                [tool.uv]
+                cache-keys = [{ file = "pyproject.toml" }, { file = "src/**/*.{h,c,hpp,cpp}" }, { file = "CMakeLists.txt" }]
+
                 [build-system]
                 requires = ["scikit-build-core>=0.10", "pybind11"]
                 build-backend = "scikit_build_core.build"

crates/uv/tests/it/init.rs

@@ -3322,6 +3322,9 @@ fn init_app_build_backend_maturin() -> Result<()> {
         python-packages = ["foo"]
         python-source = "src"
 
+        [tool.uv]
+        cache-keys = [{ file = "pyproject.toml" }, { file = "src/**/*.rs" }, { file = "Cargo.toml" }, { file = "Cargo.lock" }]
+
         [build-system]
         requires = ["maturin>=1.0,<2.0"]
         build-backend = "maturin"
@@ -3451,6 +3454,9 @@ fn init_app_build_backend_scikit() -> Result<()> {
         minimum-version = "build-system.requires"
         build-dir = "build/{wheel_tag}"
 
+        [tool.uv]
+        cache-keys = [{ file = "pyproject.toml" }, { file = "src/**/*.{h,c,hpp,cpp}" }, { file = "CMakeLists.txt" }]
+
         [build-system]
         requires = ["scikit-build-core>=0.10", "pybind11"]
         build-backend = "scikit_build_core.build"
@@ -3573,6 +3579,9 @@ fn init_lib_build_backend_maturin() -> Result<()> {
         python-packages = ["foo"]
         python-source = "src"
 
+        [tool.uv]
+        cache-keys = [{ file = "pyproject.toml" }, { file = "src/**/*.rs" }, { file = "Cargo.toml" }, { file = "Cargo.lock" }]
+
         [build-system]
         requires = ["maturin>=1.0,<2.0"]
         build-backend = "maturin"
@@ -3699,6 +3708,9 @@ fn init_lib_build_backend_scikit() -> Result<()> {
         minimum-version = "build-system.requires"
         build-dir = "build/{wheel_tag}"
 
+        [tool.uv]
+        cache-keys = [{ file = "pyproject.toml" }, { file = "src/**/*.{h,c,hpp,cpp}" }, { file = "CMakeLists.txt" }]
+
         [build-system]
         requires = ["scikit-build-core>=0.10", "pybind11"]
         build-backend = "scikit_build_core.build"