test(release): assert the release build pins bun via the version file

The release extensions workflow test asserted the literal
`bun-version: "1.3.6"` in the build job. The guard's intent is that the
release build pins Bun rather than floating, and that still holds; the
mechanism is now `bun-version-file` reading packageManager from
package.json.

Point the assertion at the new form so the guard keeps its meaning
instead of pinning a string that no longer appears.

Author: DaniAkash

packages/browseros-agent/scripts/release/release-extensions-workflow.test.ts

@@ -101,7 +101,9 @@ describe('release-extensions workflow', () => {
   it('builds and attaches the immutable CRX before optional finalization', () => {
     const build = section('  build:', '  preflight_alpha:')
     expect(build).toContain('browseros ext release')
-    expect(build).toContain('bun-version: "1.3.6"')
+    expect(build).toContain(
+      'bun-version-file: packages/browseros-agent/package.json',
+    )
     expect(build).toContain('--source-sha "$RELEASE_SHA"')
     expect(build).toContain('gh release upload')
     expect(build).toContain('needs.prepare.outputs.version')