[flags] land `enableTrustedTypesIntegration` (#35816)

## Summary This flag enables React's integration with the browser [Trusted Types API](https://developer.mozilla.org/en-US/docs/Web/API/Trusted_Types_API). The Trusted Types API is a browser security feature that helps prevent DOM-based XSS attacks. When a site enables Trusted Types enforcement via `Content-Security-Policy: require-trusted-types-for 'script'`, the browser requires that values passed to DOM injection sinks (like `innerHTML`) are typed objects (`TrustedHTML`, `TrustedScript`, `TrustedScriptURL`) created through developer-defined sanitization policies, rather than raw strings. ### What changed Previously, React always coerced values to strings (via `'' + value`) before passing them to DOM APIs like `setAttribute` and `innerHTML`. This broke Trusted Types because it converted typed objects into plain strings, which the browser would then reject under Trusted Types enforcement. React now passes values directly to DOM APIs without string coercion, preserving Trusted Types objects so the browser can validate them. This applies to `dangerouslySetInnerHTML`, all HTML and SVG attributes, and URL attributes (`href`, `action`, etc). ### Before (broken) Using Trusted Types with something like`dangerouslySetInnerHTML` would throw: ```js const sanitizer = trustedTypes.createPolicy('sanitizer', { createHTML: (input) => DOMPurify.sanitize(input), }); function Comment({text}) { const clean = sanitizer.createHTML(text); // clean is a TrustedHTML object, but React would call '' + clean, // converting it back to a plain string before setting innerHTML. // Under Trusted Types enforcement, the browser rejects the string: // // TypeError: Failed to set 'innerHTML' on 'Element': // This document requires 'TrustedHTML' assignment. return <div dangerouslySetInnerHTML={{__html: clean}} />; } ``` ### After (works) React now passes the TrustedHTML object directly to the DOM without stringifying it: ```js const policy = trustedTypes.createPolicy('sanitizer', { createHTML: (input) => DOMPurify.sanitize(input), }); function Comment({text}) { // TrustedHTML objects are passed directly to innerHTML return <div dangerouslySetInnerHTML={{__html: policy.createHTML(text)}} />; } function UserProfile({bio}) { // String attribute values also preserve Trusted Types objects return <div data-bio={policy.createHTML(bio)} />; } ``` ## Non-breaking change - Sites using Trusted Types: React no longer breaks Trusted Types enforcement. TrustedHTML and TrustedScriptURL objects passed through React props are forwarded to the DOM without being stringified. - Sites not using Trusted Types: No behavior change. DOM APIs accept both strings and Trusted Types objects, so removing the explicit string coercion is functionally identical.