refactor: restructure as installable PyPI package

- Rename app/ → fragmentapi/ for proper package naming
- Add FragmentClient class with gift_premium, gift_stars, topup_ton methods
- Restructure core/ → types/ (exceptions, results, constants)
- Merge utils/hash.py into utils/client.py
- Replace _version.py with importlib.metadata
- Add input validation with min/max bounds
- Add unit tests: decode, client init/cookies
- Rename 002_test_hash → 003_test_hash
- Clean up pyproject.toml: pin deps, production classifiers

tests/001_test_decode.py

@@ -1,11 +1,11 @@
-"""Tests for clean_decode() — BOC-encoded Fragment payloads decode to
-human-readable UTF-8 with the Telegram label and Ref# intact."""
+"""Tests for clean_decode() — BOC-encoded Fragment payloads decode to UTF-8."""
 
 import re
 
 import pytest
 
-from app.utils.decoder import clean_decode
+from fragmentapi.types import RequestError
+from fragmentapi.utils.decoder import clean_decode
 
 PAYLOADS = [
     pytest.param(
@@ -24,12 +24,17 @@ PAYLOADS = [
 
 
 @pytest.mark.parametrize("payload", PAYLOADS)
-def test_payload(payload: str) -> None:
+def test_decode_payload(payload: str) -> None:
     result = clean_decode(payload)
     assert "Telegram" in result
     assert re.search(r"Ref#[A-Za-z0-9]+", result), f"no Ref# in {result!r}"
-    assert all(ord(c) <= 127 for c in result), f"non-ASCII chars in {result!r}"
+    assert all(ord(c) < 128 for c in result), f"non-ASCII chars in {result!r}"
 
 
-def test_empty_input_returns_string() -> None:
-    assert isinstance(clean_decode(""), str)
+def test_empty_payload_returns_empty_string() -> None:
+    assert clean_decode("") == ""
+
+
+def test_invalid_payload_raises_request_error() -> None:
+    with pytest.raises(RequestError):
+        clean_decode("!!!not-valid-base64!!!")