refactor: harden client, clean tests, and fix timeouts

- Security & validation: tighten hash regex, add HTTP timeouts to all
  requests, pass client.timeout through get_fragment_hash and AsyncClient
- Constants: move all constants to types/constants.py; remove re-exports
  from types/__init__.py; add DEFAULT_TIMEOUT, REQUIRED_COOKIE_KEYS
- FragmentClient: add timeout param (default 30 s); async-context-manager
  support; remove WALLET_CLASSES from public API
- Exceptions: remove dead INVALID_USERNAME constant (Fragment validates
  server-side); keep full hierarchy intact
- Tests: add 006_test_methods_mock.py (6 mock tests for all 3 methods);
  DRY-refactor 004_test_balance.py (_patch_wallet context manager);
  clean up 005_test_methods.py (remove fragile network test, rename tests)
- Examples: switch all 4 examples to async-with; align error messages;
  replace %-format with f-strings
- README: rewrite usage section with single comprehensive async-with
  example covering all 3 methods and full exception hierarchy
- CI: add mypy step to lint job; add pytest-mock and mypy to dev deps;
  set FORCE_JAVASCRIPT_ACTIONS_TO_NODE24 on all jobs; fix COOKIES_JSON
  to job-level env var

.github/workflows/ci.yml

@@ -10,6 +10,8 @@ jobs:
   lint:
     name: Lint & Format
     runs-on: ubuntu-latest
+    env:
+      FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
 
     steps:
       - uses: actions/checkout@v6.0.2
@@ -26,9 +28,14 @@ jobs:
 
       - run: black --check . --target-version py312
 
+      - run: mypy pyfragment
+
   test:
     name: Tests
     runs-on: ubuntu-latest
+    env:
+      FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
+      COOKIES_JSON: ${{ secrets.COOKIES_JSON }}
 
     steps:
       - uses: actions/checkout@v6.0.2
@@ -41,10 +48,4 @@ jobs:
 
       - run: uv pip install --system ".[dev]"
 
-      - name: Write cookies.json
-        if: ${{ env.COOKIES_JSON != '' }}
-        run: echo "$COOKIES_JSON" > cookies.json
-        env:
-          COOKIES_JSON: ${{ secrets.COOKIES_JSON }}
-
       - run: pytest