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

tests/conftest.py

@@ -1,17 +1,16 @@
 import json
-from pathlib import Path
+import os
 
 import pytest
 
 
 @pytest.fixture
 def cookies():
-    """Load Fragment cookies from cookies.json; skip the test if unavailable."""
-    cookies_path = Path(__file__).resolve().parents[1] / "cookies.json"
-    if not cookies_path.exists():
-        pytest.skip("cookies.json not found")
+    """Load Fragment cookies from COOKIES_JSON env var; skip if unavailable."""
+    raw = os.environ.get("COOKIES_JSON")
+    if not raw:
+        pytest.skip("COOKIES_JSON env var not set")
     try:
-        with cookies_path.open("r", encoding="utf-8") as f:
-            return json.load(f)
+        return json.loads(raw)
     except Exception as exc:
         pytest.skip(f"Cookies unavailable — {exc}")