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_filter.py

@@ -1,6 +1,8 @@
+"""Tests for twitter_cli.filter module."""
+
 from __future__ import annotations
 
-from twitter_cli.filter import filter_tweets
+from twitter_cli.filter import filter_tweets, score_tweet
 
 
 def test_filter_tweets_does_not_mutate_input(tweet_factory) -> None:
@@ -29,3 +31,58 @@ def test_filter_tweets_applies_language_and_retweet_filters(tweet_factory) -> No
     )
 
     assert [tweet.id for tweet in output] == ["1"]
+
+
+def test_filter_topn_mode(tweet_factory) -> None:
+    tweets = [tweet_factory(str(i)) for i in range(10)]
+    output = filter_tweets(tweets, {"mode": "topN", "topN": 3})
+    assert len(output) == 3
+
+
+def test_filter_topn_default(tweet_factory) -> None:
+    """Default topN is 20, so 5 tweets should all be returned."""
+    tweets = [tweet_factory(str(i)) for i in range(5)]
+    output = filter_tweets(tweets, {"mode": "topN"})
+    assert len(output) == 5
+
+
+def test_filter_score_mode(tweet_factory) -> None:
+    from twitter_cli.models import Metrics
+
+    tweets = [
+        tweet_factory("high", metrics=Metrics(likes=1000, retweets=500, replies=200, views=100000, bookmarks=50)),
+        tweet_factory("low", metrics=Metrics(likes=0, retweets=0, replies=0, views=1, bookmarks=0)),
+    ]
+    output = filter_tweets(tweets, {"mode": "score", "minScore": 100.0})
+    assert len(output) == 1
+    assert output[0].id == "high"
+
+
+def test_filter_empty_input() -> None:
+    output = filter_tweets([], {"mode": "all"})
+    assert output == []
+
+
+def test_score_tweet_basic(tweet_factory) -> None:
+    tweet = tweet_factory("1")
+    score = score_tweet(tweet)
+    assert isinstance(score, float)
+    assert score > 0
+
+
+def test_score_tweet_custom_weights(tweet_factory) -> None:
+    tweet = tweet_factory("1")
+    weights = {"likes": 0, "retweets": 0, "replies": 0, "bookmarks": 0, "views_log": 0}
+    assert score_tweet(tweet, weights) == 0.0
+
+
+def test_filter_all_mode_sorts_by_score(tweet_factory) -> None:
+    from twitter_cli.models import Metrics
+
+    tweets = [
+        tweet_factory("low", metrics=Metrics(likes=1)),
+        tweet_factory("high", metrics=Metrics(likes=100)),
+    ]
+    output = filter_tweets(tweets, {"mode": "all"})
+    assert output[0].id == "high"
+    assert output[1].id == "low"