refactor: unify exception handling, add ISO 8601 time, dedup commands, expand tests

- Replace _error_code_for_message() string matching with error_code attribute on exception classes
- Add error_code to all TwitterError subclasses (AuthenticationError, RateLimitError, etc.)
- Add InvalidInputError exception class
- TwitterAPIError derives error_code from HTTP status code automatically
- auth.py: use AuthenticationError instead of RuntimeError
- cli.py: catch (TwitterError, RuntimeError) for backward compat
- Extract _fetch_and_display_users() to deduplicate followers/following commands
- Add format_iso8601() to timeutil.py
- Add createdAtISO field to tweet and user profile serialization
- New test files: test_output.py, test_cache.py, test_timeutil.py
- Expand test_filter.py (topN, score mode, custom weights, empty input)
- Tests: 152 → 194 unit tests, all passing

tests/test_cache.py

@@ -0,0 +1,101 @@
+"""Tests for twitter_cli.cache module."""
+
+from __future__ import annotations
+
+import json
+import time
+
+
+from twitter_cli.cache import resolve_cached_tweet, save_tweet_cache
+from twitter_cli.models import Author, Metrics, Tweet
+
+
+def _make_tweet(tweet_id: str, text: str = "hello") -> Tweet:
+    return Tweet(
+        id=tweet_id,
+        text=text,
+        author=Author(id="u1", name="Alice", screen_name="alice"),
+        metrics=Metrics(likes=1),
+        created_at="2025-01-01",
+    )
+
+
+class TestSaveAndResolve:
+    """save_tweet_cache → resolve_cached_tweet round-trip."""
+
+    def test_round_trip(self, tmp_path, monkeypatch) -> None:
+        cache_file = tmp_path / "last_results.json"
+        monkeypatch.setattr("twitter_cli.cache._CACHE_FILE", cache_file)
+        monkeypatch.setattr("twitter_cli.cache._CACHE_DIR", tmp_path)
+
+        tweets = [_make_tweet("100"), _make_tweet("200"), _make_tweet("300")]
+        save_tweet_cache(tweets)
+
+        assert cache_file.exists()
+        tweet_id, size = resolve_cached_tweet(1)
+        assert tweet_id == "100"
+        assert size == 3
+
+        tweet_id, size = resolve_cached_tweet(3)
+        assert tweet_id == "300"
+        assert size == 3
+
+    def test_out_of_range_returns_none(self, tmp_path, monkeypatch) -> None:
+        cache_file = tmp_path / "last_results.json"
+        monkeypatch.setattr("twitter_cli.cache._CACHE_FILE", cache_file)
+        monkeypatch.setattr("twitter_cli.cache._CACHE_DIR", tmp_path)
+
+        save_tweet_cache([_make_tweet("100")])
+
+        tweet_id, size = resolve_cached_tweet(99)
+        assert tweet_id is None
+        assert size == 1
+
+
+class TestCacheExpiry:
+    """TTL expiration behavior."""
+
+    def test_expired_cache_returns_none(self, tmp_path, monkeypatch) -> None:
+        cache_file = tmp_path / "last_results.json"
+        monkeypatch.setattr("twitter_cli.cache._CACHE_FILE", cache_file)
+
+        # Write cache with old timestamp
+        payload = {
+            "created_at": time.time() - 7200,  # 2 hours ago
+            "tweets": [{"index": 1, "id": "100", "author": "alice", "text": "hi"}],
+        }
+        cache_file.write_text(json.dumps(payload), encoding="utf-8")
+
+        tweet_id, size = resolve_cached_tweet(1)
+        assert tweet_id is None
+        assert size == 0
+
+
+class TestCacheEdgeCases:
+    """Corrupted and missing cache files."""
+
+    def test_missing_file(self, tmp_path, monkeypatch) -> None:
+        cache_file = tmp_path / "does_not_exist.json"
+        monkeypatch.setattr("twitter_cli.cache._CACHE_FILE", cache_file)
+
+        tweet_id, size = resolve_cached_tweet(1)
+        assert tweet_id is None
+        assert size == 0
+
+    def test_corrupted_json(self, tmp_path, monkeypatch) -> None:
+        cache_file = tmp_path / "last_results.json"
+        cache_file.write_text("{{invalid json", encoding="utf-8")
+        monkeypatch.setattr("twitter_cli.cache._CACHE_FILE", cache_file)
+
+        tweet_id, size = resolve_cached_tweet(1)
+        assert tweet_id is None
+        assert size == 0
+
+    def test_wrong_structure(self, tmp_path, monkeypatch) -> None:
+        cache_file = tmp_path / "last_results.json"
+        cache_file.write_text('"just a string"', encoding="utf-8")
+        monkeypatch.setattr("twitter_cli.cache._CACHE_FILE", cache_file)
+
+        tweet_id, size = resolve_cached_tweet(1)
+        assert tweet_id is None
+        assert size == 0