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/get_wallet.py

@@ -1,5 +1,5 @@
 """
-Example: initializing FragmentClient.
+Example: fetch wallet address, state, and balance.
 
 Cookies can be passed as a dict or as a JSON string.
 wallet_version defaults to "V5R1" — change to "V4R2" for older wallets.
@@ -9,7 +9,7 @@ import asyncio
 
 from pyfragment import FragmentClient
 
-SEED = "word1 word2 word3 word4 word5 word6 word7 word8 word9 word10 word11 word12 word13 word14 word15 word16 word17 word18 word19 word20 word21 word22 word23 word24"
+SEED = "word1 word2 ... word24"
 API_KEY = "YOUR_TONAPI_KEY"
 COOKIES = {
     "stel_ssid": "YOUR_STEL_SSID",
@@ -20,20 +20,16 @@ COOKIES = {
 
 
 async def main() -> None:
-    client = FragmentClient(
+    async with FragmentClient(
         seed=SEED,
         api_key=API_KEY,
         cookies=COOKIES,
         wallet_version="V5R1",  # or "V4R2"
-    )
-
-    wallet = await client.get_wallet()
-
-    print("FragmentClient initialized")
-    print("   %-16s %s" % ("Wallet version:", client.wallet_version))
-    print("   %-16s %s" % ("Address:", wallet.address))
-    print("   %-16s %s" % ("State:", wallet.state))
-    print("   %-16s %s TON" % ("Balance:", wallet.balance))
+    ) as client:
+        wallet = await client.get_wallet()
+        print(f"Address: {wallet.address}")
+        print(f"State:   {wallet.state}")
+        print(f"Balance: {wallet.balance} TON")
 
 
 if __name__ == "__main__":