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

examples/purchase_premium.py

@@ -23,22 +23,17 @@ MONTHS = 3  # 3, 6, or 12
 
 
 async def main() -> None:
-    client = FragmentClient(seed=SEED, api_key=API_KEY, cookies=COOKIES)
+    async with FragmentClient(seed=SEED, api_key=API_KEY, cookies=COOKIES) as client:
+        try:
+            result = await client.purchase_premium(USERNAME, months=MONTHS, show_sender=True)
+        except UserNotFoundError:
+            print(f"User {USERNAME} was not found on fragment.com — check the username and try again.")
+            return
+        except ConfigurationError as e:
+            print(f"Invalid argument: {e}")
+            return
 
-    try:
-        result = await client.purchase_premium(USERNAME, months=MONTHS, show_sender=True)
-    except UserNotFoundError:
-        print(f"User {USERNAME!r} not found on Fragment.")
-        return
-    except ConfigurationError as e:
-        print(f"Invalid parameters: {e}")
-        return
-
-    print("Premium purchased")
-    print("   %-14s %s" % ("Username:", result.username))
-    print("   %-14s %s months" % ("Duration:", result.months))
-    print("   %-14s %s" % ("Transaction:", result.transaction_id))
-    print("   %-14s %s" % ("Timestamp:", result.timestamp))
+    print(f"{result.months} months of Premium successfully sent to {result.username} | tx: {result.transaction_id}")
 
 
 if __name__ == "__main__":