Escape regex metacharacters in siteApiNamespace before interpolating into RegExp

Summary

In src/utils/api-fetch.js, apiPathModifierMiddleware interpolates siteApiNamespace values directly into new RegExp() without escaping regex metacharacters:

const namespaceRegex = new RegExp( `(${ siteApiNamespace.join( '|' ) })` );

If a namespace contained characters like ., +, (, ), *, etc., they would be interpreted as regex syntax rather than matched literally.

Risk

Low — siteApiNamespace comes from the native app bridge config (not user input), so exploitation requires a compromised host app. But it's a latent correctness bug: a namespace like wp/v2.1 would match wp/v2X (. matches any character).

Suggested fix

Escape each namespace before joining:

const escaped = siteApiNamespace.map( ( ns ) =>
    ns.replace( /[.*+?^${}()|[\]\\]/g, '\\$&' )
);
const namespaceRegex = new RegExp( `(${ escaped.join( '|' ) })` );

Existing tests in src/utils/api-fetch.test.js pass with this change.

Found during adversarial code review of #TBD.

Start in src/utils/api-fetch.js at apiPathModifierMiddleware, then review the related cases in src/utils/api-fetch.test.js. Verify that namespaces containing regex metacharacters are matched literally and that the existing tests still pass; add coverage for the reported namespace example if needed.