fix(backend): Make URL pinning work with extra_url_validator (#9940)

Github Blocks use an URL transformer passed to `Requests` to convert web
URLs to the API URLs. This doesn't always work with the anti-SSRF URL
pinning mechanism that was implemented in #8531.

### Changes 🏗️
In `Requests.request(..)`:
- Apply `validate_url` *after* `extra_url_validator`, to prevent
mismatch between `pinned_url` and `original_hostname`
- Simplify logic & add clarifying comments

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Tested the github blocks that had the issue

---------

Co-authored-by: Reinier van der Leer <pwuts@agpt.co>

.vscode/launch.json

@@ -32,9 +32,9 @@
       "type": "debugpy",
       "request": "launch",
       "module": "backend.app",
-      // "env": {
-      //   "ENV": "dev"
-      // },
+      "env": {
+        "OBJC_DISABLE_INITIALIZE_FORK_SAFETY": "YES"
+      },
       "envFile": "${workspaceFolder}/backend/.env",
       "justMyCode": false,
       "cwd": "${workspaceFolder}/autogpt_platform/backend"