fix(typescript/prefer-namespace-keyword): skip nested TSModuleDeclarations (#15806)

Prevent fix replacing text within the module name or block in the case of nested modules.

Before this PR:

Input:

```ts
module X.Y.module {
  x = "module";
}
```

After fix:

```ts
namespace X.Y.namespace {
  x = "namespace";
}
```

Prevent this by exiting early if the `TSModuleDeclaration` is nested (i.e. `Y` or `module` in above example).

Note: I discovered this bug when I replaced early `return` with `unwrap` in #15805. IMO it's better to `unwrap` when you believe the check cannot fail, as the panic may alert us to bugs which may otherwise lie undiscovered.

Side note: The design we have with nested `TSModuleDeclaration`s representing `module X.Y.Z {}` is pretty unintutive. I'd like to change it if we can.

Author: overlookmotel

crates/oxc_linter/src/rules/typescript/prefer_namespace_keyword.rs

@@ -61,6 +61,12 @@ impl Rule for PreferNamespaceKeyword {
             return;
         }
 
+        // Ignore nested `TSModuleDeclaration`s
+        // e.g. the 2 inner `TSModuleDeclaration`s in `module A.B.C {}`
+        if let AstKind::TSModuleDeclaration(_) = ctx.nodes().parent_kind(node.id()) {
+            return;
+        }
+
         let Some(offset) = ctx.find_next_token_from(module.span.start, "module") else { return };
 
         ctx.diagnostic_with_fix(prefer_namespace_keyword_diagnostic(module.span), |fixer| {
@@ -133,6 +139,11 @@ fn test() {
             None,
         ),
         ("declare /* module */ module foo {}", "declare /* module */ namespace foo {}", None),
+        (
+            "declare module X.Y.module { x = 'module'; }",
+            "declare namespace X.Y.module { x = 'module'; }",
+            None,
+        ),
     ];
 
     Tester::new(PreferNamespaceKeyword::NAME, PreferNamespaceKeyword::PLUGIN, pass, fail)