rust: doctest: add LocalModule fallback for #[vtable] ThisModule

Add a `LocalModule` struct with a null-pointer `ModuleMetadata` impl
in the doctest harness, so that `crate::LocalModule` (auto-inserted
by `#[vtable]`) resolves correctly when there is no `module!` macro.

Assisted-by: opencode:glm-5.2
Reviewed-by: Andreas Hindborg <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
Acked-by: Danilo Krummrich <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alvin Sun <[email protected]>
Link: https://patch.msgid.link/[email protected]
[ Fixed `clippy::undocumented_unsafe_blocks` lint by wrapping with
  a block. Added interim `#[allow(dead_code)]`. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

scripts/rustdoc_test_gen.rs

@@ -239,6 +239,25 @@ pub extern "C" fn {kunit_name}(__kunit_test: *mut ::kernel::bindings::kunit) {{
 
 const __LOG_PREFIX: &[u8] = b"rust_doctests_kernel\0";
 
+/// Dummy module type for doctest context.
+#[allow(dead_code)]
+struct LocalModule;
+
+use kernel::{{
+    str::CStr,
+    ModuleMetadata,
+    ThisModule, //
+}};
+use core::ptr::null_mut;
+
+impl ModuleMetadata for LocalModule {{
+    const NAME: &'static CStr = c"rust_doctests_kernel";
+    const THIS_MODULE: ThisModule = {{
+        // SAFETY: `try_module_get`/`module_put` handle null module pointers gracefully.
+        unsafe {{ ThisModule::from_ptr(null_mut()) }}
+    }};
+}}
+
 {rust_tests}
 "#
     )