pass the source map option in watch mode (#353)

following is missing when generating file in watch mode:
```//# sourceMappingURL=csharp-codegen.js.map```

<img width="3402" height="260" alt="image" src="https://github.com/user-attachments/assets/126cc552-fbd8-4541-8582-20ea66cc1624" />

Author: RodgeFu

.chronus/changes/fix-mapfile-for-watch-2026-0-25-14-1-24.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@alloy-js/cli"
+---
+
+Make sure sourceMappingURL is added in --watch mode

packages/cli/src/cli.ts

@@ -101,7 +101,10 @@ function watchMain() {
           .map((x) => x.fileName),
         opts.rootDir,
         opts.outDir,
-        { addSourceInfo },
+        {
+          sourceMaps: opts.options.sourceMap,
+          addSourceInfo,
+        },
       );
     } catch (e) {
       // eslint-disable-next-line no-console