pkgs/cli/vars: Add dependency validation

Add explicit dependency validation to vars, so that proper error
messages can be surfaced to the user.

Instead of:
```
Traceback (most recent call last):
  File "/home/lhebendanz/Projects/clan-core/pkgs/clan-cli/clan_lib/async_run/__init__.py", line 154, in run
    self.result = AsyncResult(_result=self.function(*self.args, **self.kwargs))
                                      ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lhebendanz/Projects/clan-core/pkgs/clan-cli/clan_cli/machines/update.py", line 62, in run_update_wit
h_network
    run_machine_update(
    ~~~~~~~~~~~~~~~~~~^
        machine=machine,
        ^^^^^^^^^^^^^^^^
    ...<2 lines>...
        upload_inputs=upload_inputs,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/lhebendanz/Projects/clan-core/pkgs/clan-cli/clan_lib/machines/update.py", line 158, in run_machine_u
pdate
    run_generators([machine], generators=None, full_closure=False)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/lhebendanz/Projects/clan-core/pkgs/clan-cli/clan_lib/vars/generate.py", line 156, in run_generators
    all_generators = get_generators(machines, full_closure=True)
  File "/home/lhebendanz/Projects/clan-core/pkgs/clan-cli/clan_lib/vars/generate.py", line 50, in get_generators
    all_generators_list = Generator.get_machine_generators(
        all_machines,
        flake,
        include_previous_values=include_previous_values,
    )
  File "/home/lhebendanz/Projects/clan-core/pkgs/clan-cli/clan_cli/vars/generator.py", line 246, in get_machine_ge
nerators
    if generators_data[dep]["share"]
       ~~~~~~~~~~~~~~~^^^^^
KeyError: 'bla'
```

We now get:
```
$> Generator 'my_generator' on machine 'my_machine' depends on generator 'non_existing_generator', but 'non_existing_generator' does not exist
```

Closes: #5698

pkgs/clan-cli/clan_cli/tests/test_vars.py

@@ -766,6 +766,28 @@ def test_prompt(
     assert sops_store.get(my_generator, "prompt_persist").decode() == "prompt_persist"
 
 
+@pytest.mark.with_core
+def test_non_existing_dependency_raises_error(
+    monkeypatch: pytest.MonkeyPatch,
+    flake_with_sops: ClanFlake,
+) -> None:
+    """Ensure that a generator with a non-existing dependency raises a clear error."""
+    flake = flake_with_sops
+
+    config = flake.machines["my_machine"] = create_test_machine_config()
+    my_generator = config["clan"]["core"]["vars"]["generators"]["my_generator"]
+    my_generator["files"]["my_value"]["secret"] = False
+    my_generator["script"] = 'echo "$RANDOM" > "$out"/my_value'
+    my_generator["dependencies"] = ["non_existing_generator"]
+    flake.refresh()
+    monkeypatch.chdir(flake.path)
+    with pytest.raises(
+        ClanError,
+        match="Generator 'my_generator' on machine 'my_machine' depends on generator 'non_existing_generator', but 'non_existing_generator' does not exist",
+    ):
+        cli.run(["vars", "generate", "--flake", str(flake.path), "my_machine"])
+
+
 @pytest.mark.with_core
 def test_shared_vars_must_never_depend_on_machine_specific_vars(
     monkeypatch: pytest.MonkeyPatch,

pkgs/clan-cli/clan_cli/vars/generator.py

@@ -66,6 +66,41 @@ class Generator:
     _public_store: "StoreBase | None" = None
     _secret_store: "StoreBase | None" = None
 
+    @staticmethod
+    def validate_dependencies(
+        generator_name: str,
+        machine_name: str,
+        dependencies: list[str],
+        generators_data: dict[str, dict],
+    ) -> list[GeneratorKey]:
+        """Validate and build dependency keys for a generator.
+
+        Args:
+            generator_name: Name of the generator that has dependencies
+            machine_name: Name of the machine the generator belongs to
+            dependencies: List of dependency generator names
+            generators_data: Dictionary of all available generators for this machine
+
+        Returns:
+            List of GeneratorKey objects
+
+        Raises:
+            ClanError: If a dependency does not exist
+
+        """
+        deps_list = []
+        for dep in dependencies:
+            if dep not in generators_data:
+                msg = f"Generator '{generator_name}' on machine '{machine_name}' depends on generator '{dep}', but '{dep}' does not exist. Please check your configuration."
+                raise ClanError(msg)
+            deps_list.append(
+                GeneratorKey(
+                    machine=None if generators_data[dep]["share"] else machine_name,
+                    name=dep,
+                )
+            )
+        return deps_list
+
     @property
     def key(self) -> GeneratorKey:
         if self.share:
@@ -240,15 +275,12 @@ class Generator:
                     name=gen_name,
                     share=share,
                     files=files,
-                    dependencies=[
-                        GeneratorKey(
-                            machine=None
-                            if generators_data[dep]["share"]
-                            else machine_name,
-                            name=dep,
-                        )
-                        for dep in gen_data["dependencies"]
-                    ],
+                    dependencies=cls.validate_dependencies(
+                        gen_name,
+                        machine_name,
+                        gen_data["dependencies"],
+                        generators_data,
+                    ),
                     migrate_fact=gen_data.get("migrateFact"),
                     validation_hash=gen_data.get("validationHash"),
                     prompts=prompts,