fix(isolated-declarations): incorrect nested namespace output in isolated declarations (#15800)

Dotted namespace syntax `export namespace X.Y.Z {}` was incorrectly transformed to `export declare namespace Y.Z.Z {}` instead of `export declare namespace X.Y.Z {}`.

**Root cause:** Variable shadowing in `transform_ts_module_declaration`. When processing nested `TSModuleDeclaration` bodies, the pattern match variable `decl` shadowed the function parameter, causing the inner namespace name to be used instead of the outer one.

**Changes:**
- Renamed pattern match variable from `decl` to `inner_decl` to preserve access to outer declaration's identifier
- Added test coverage for dotted namespace syntax with varying nesting depths

**Example:**
```ts
// Input
export namespace X.Y.Z {}

// Before: Y.Z.Z (incorrect)
// After: X.Y.Z (correct)
export declare namespace X.Y.Z {
  export {};
}
```

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<details>

<summary>Original prompt</summary>

>
> ----
>
> *This section details on the original issue you should resolve*
>
> <issue_title>Isolated declarations: Incorrect output for nested `namespace`</issue_title>
> <issue_description>Input:
>
> ```ts
> //               _____
> export namespace X.Y.Z {}
> ```
>
> Isolated declarations output:
>
> ```ts
> //                       _____
> export declare namespace Y.Z.Z {
>   export {};
> }
> ```
>
> [Playground](https://playground.oxc.rs/#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)
>
> Maybe this code is the cause?
>
> https://github.com/oxc-project/oxc/blob/4608549fc7715ee76c4a6d7a78069ff339f52267/crates/oxc_isolated_declarations/src/lib.rs#L521-L523
>
> Looks fishy! Shouldn't it walk down to find the nested `TSModuleBlock` when `decl.body` is a `TSModuleDeclarationBody::TSModuleDeclaration`?</issue_description>
>
> ## Comments on the Issue (you are @copilot in this section)
>
> <comments>
> </comments>
>

</details>

- Fixes #15729

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Author: Copilot

crates/oxc_isolated_declarations/src/declaration.rs

@@ -129,8 +129,8 @@ impl<'a> IsolatedDeclarations<'a> {
         // Follows https://github.com/microsoft/TypeScript/pull/54134
         let kind = TSModuleDeclarationKind::Namespace;
         match body {
-            TSModuleDeclarationBody::TSModuleDeclaration(decl) => {
-                let inner = self.transform_ts_module_declaration(decl);
+            TSModuleDeclarationBody::TSModuleDeclaration(inner_decl) => {
+                let inner = self.transform_ts_module_declaration(inner_decl);
                 self.ast.alloc_ts_module_declaration(
                     decl.span,
                     decl.id.clone_in(self.ast.allocator),

crates/oxc_isolated_declarations/tests/fixtures/nested-namespace-dotted.ts

@@ -0,0 +1,15 @@
+// Dotted namespace syntax should preserve all names correctly
+export namespace X.Y.Z {}
+
+// Nested dotted namespaces with content
+export namespace A.B.C {
+    export const value = 1;
+    export function foo(): void {}
+}
+
+// Deeply nested
+export namespace Deep.Nested.Namespace.Structure {
+    export interface Config {
+        value: string;
+    }
+}

crates/oxc_isolated_declarations/tests/snapshots/nested-namespace-dotted.snap

@@ -0,0 +1,22 @@
+---
+source: crates/oxc_isolated_declarations/tests/mod.rs
+input_file: crates/oxc_isolated_declarations/tests/fixtures/nested-namespace-dotted.ts
+---
+```
+==================== .D.TS ====================
+
+// Dotted namespace syntax should preserve all names correctly
+export declare namespace X.Y.Z {
+	export {};
+}
+// Nested dotted namespaces with content
+export declare namespace A.B.C {
+	const value = 1;
+	function foo(): void;
+}
+// Deeply nested
+export declare namespace Deep.Nested.Namespace.Structure {
+	interface Config {
+		value: string;
+	}
+}