Fix XMLHttpRequest prototype-key headers

[OskarEichler]

Summary:

React Native XMLHttpRequest stores request and normalized response headers in ordinary objects, so valid names such as constructor, __proto__, and toString collide with inherited properties. Use prototype-free header maps on reset and response normalization, simplify normalization to a direct loop, and cover request combination plus response lookup behavior.

Fixes #58232.

Changelog:

[GENERAL] [FIXED] - Support XMLHttpRequest header names that overlap Object prototype properties.

Test Plan:

• Exact upstream suite: 14 existing tests passed and both new regressions failed.
• Fixed focused Jest suite: 16/16 passed.
• Fresh Flow check: 0 errors.
• Targeted no-ignore ESLint, Prettier, and git diff --check passed.

No UI change; screenshots are not applicable.