fix(form): honor encType when selecting request body - #13997

Proposed Changes

When Form receives both encType and a Content-Type header, it can send a body using a different encoding from the final header. For example, encType="application/x-www-form-urlencoded" with a JSON header sends a JSON body under a URL-encoded content type. A differently cased header can also be duplicated when Fetch normalizes the names.

Use the existing non-multipart encType precedence for both body selection and the final header, replacing overridden header names case-insensitively. Preserve multipart boundary handling, other headers, and the caller-provided headers object.

Add 18 regression and compatibility cases for JSON, URL-encoded and text bodies, mixed-case headers, and multipart combinations. Ten new cases fail against the original implementation.

Validation with Node 22:

• 133 suites, 1,850 unit tests and 26 snapshots passed.
• All 90 existing Chromium E2E tests passed with one worker and no retries, using a local browser/server configuration. Test files and assertions are unchanged.
• pnpm type, pnpm test:type, lint, Prettier, builds, and both API Extractor checks passed. No added lint warnings; the API report is unchanged.

Type of change

• Bug fix

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes