chore(deps): remove dead undici HTTP/2 adapter and dependency

The SDK's HTTP/2 transport is the native `node:http2` implementation
(`makeHttp2Fetch` → `createH2Fetch`). The old undici-based adapter,
`src/lib/undici-fetch.ts`, is no longer imported anywhere — it's dead code, and
`undici` is a dependency only that file used.

Remove the dead adapter, its test, and the `undici` dependency. This also drops
undici 7's transitive Node >= 20.18.1 install constraint.

Stale undici references in doc comments are cleaned up separately in the
HTTP/2-default PR (#815).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Reflex

package.json

@@ -48,7 +48,6 @@
     "formdata-node": "^4.3.2",
     "node-fetch": "^2.6.7",
     "tar": "^7.5.2",
-    "undici": "^7.26.0",
     "uuidv7": "^1.0.2",
     "zod": "^3.24.1"
   },

src/lib/undici-fetch.ts

@@ -3844,11 +3844,6 @@ undici-types@~5.26.4:
   resolved "https://registry.npmjs.org/undici-types/-/undici-types-5.26.5.tgz"
   integrity sha512-JlCMO+ehdEIKqlFxk6IfVoAUVmgz7cU7zD/h9XZ0qzeosSHmUJVOzSQvvYSYWXkFXC+IfLKSIffhv0sVZup6pA==
 
-undici@^7.26.0:
-  version "7.26.0"
-  resolved "https://registry.yarnpkg.com/undici/-/undici-7.26.0.tgz#d413a2b5752e3e71e003bb268dec32b9a0ad0ce7"
-  integrity sha512-3O9Tf67pGhgOv9jM35AbhkXAKi13f3oy3aE4CSgr+TckGeY+/iu97ZXN+J7DpHPzLbVApFd1IFhcnBjREYXYcg==
-
 universalify@^2.0.0:
   version "2.0.1"
   resolved "https://registry.npmjs.org/universalify/-/universalify-2.0.1.tgz"