Pages Router: literal `<form action='action:...'>` string is stripped instead of rendered as-is

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

Pages Router pages with a literal action='action:foo' (a fake/string-typed action attribute, not a real server action reference) should keep the string in the form's action attribute. vinext strips it, leaving an empty string.

Expected `action="action:foo"`, received `action=""`

Estimated Impact

~1 test failures across the deploy suite.

Affected Test Suites

• test/e2e/app-dir/action-in-pages-router/action-in-pages-router.test.ts

Recommendation

1. Reproduce first in vinext's own test suite. Render a Pages Router page with <form action="action:foo"> and assert the action attribute survives serialisation.

2. Stop treating arbitrary action: strings as server action references. Server-action detection in the Pages Router pipeline should only match real action references, not arbitrary strings starting with action:.