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_output.py

@@ -0,0 +1,147 @@
+"""Tests for twitter_cli.output module."""
+
+from __future__ import annotations
+
+import json
+
+import click
+import pytest
+import yaml
+
+from twitter_cli.output import (
+    default_structured_format,
+    emit_structured,
+    ensure_utf8_streams,
+    error_payload,
+    success_payload,
+    use_rich_output,
+)
+
+
+# ── ensure_utf8_streams ─────────────────────────────────────────────────
+
+
+def test_ensure_utf8_streams_no_error() -> None:
+    """ensure_utf8_streams should not raise on any platform."""
+    ensure_utf8_streams()
+
+
+# ── success_payload / error_payload ──────────────────────────────────────
+
+
+def test_success_payload_structure() -> None:
+    payload = success_payload({"key": "value"})
+    assert payload["ok"] is True
+    assert payload["schema_version"] == "1"
+    assert payload["data"] == {"key": "value"}
+
+
+def test_success_payload_with_list() -> None:
+    payload = success_payload([1, 2, 3])
+    assert payload["ok"] is True
+    assert payload["data"] == [1, 2, 3]
+
+
+def test_error_payload_structure() -> None:
+    payload = error_payload("not_found", "User not found")
+    assert payload["ok"] is False
+    assert payload["schema_version"] == "1"
+    assert payload["error"]["code"] == "not_found"
+    assert payload["error"]["message"] == "User not found"
+    assert "details" not in payload["error"]
+
+
+def test_error_payload_with_details() -> None:
+    payload = error_payload("api_error", "oops", details={"id": "123"})
+    assert payload["error"]["details"] == {"id": "123"}
+
+
+# ── default_structured_format ────────────────────────────────────────────
+
+
+def test_format_json_flag() -> None:
+    assert default_structured_format(as_json=True, as_yaml=False) == "json"
+
+
+def test_format_yaml_flag() -> None:
+    assert default_structured_format(as_json=False, as_yaml=True) == "yaml"
+
+
+def test_format_both_flags_raises() -> None:
+    with pytest.raises(click.UsageError):
+        default_structured_format(as_json=True, as_yaml=True)
+
+
+def test_format_env_yaml(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "yaml")
+    assert default_structured_format(as_json=False, as_yaml=False) == "yaml"
+
+
+def test_format_env_json(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "json")
+    assert default_structured_format(as_json=False, as_yaml=False) == "json"
+
+
+def test_format_env_rich(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "rich")
+    assert default_structured_format(as_json=False, as_yaml=False) is None
+
+
+def test_format_auto_non_tty(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "auto")
+    monkeypatch.setattr("sys.stdout", type("FakeStdout", (), {"isatty": lambda self: False})())
+    assert default_structured_format(as_json=False, as_yaml=False) == "yaml"
+
+
+# ── use_rich_output ──────────────────────────────────────────────────────
+
+
+def test_use_rich_when_no_structured(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "rich")
+    assert use_rich_output(as_json=False, as_yaml=False) is True
+
+
+def test_no_rich_when_json() -> None:
+    assert use_rich_output(as_json=True, as_yaml=False) is False
+
+
+def test_no_rich_when_compact(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "rich")
+    assert use_rich_output(as_json=False, as_yaml=False, compact=True) is False
+
+
+# ── emit_structured ──────────────────────────────────────────────────────
+
+
+def test_emit_structured_json(capsys) -> None:
+    result = emit_structured({"key": "val"}, as_json=True, as_yaml=False)
+    assert result is True
+    captured = capsys.readouterr()
+    parsed = json.loads(captured.out)
+    assert parsed["ok"] is True
+    assert parsed["data"]["key"] == "val"
+
+
+def test_emit_structured_yaml(capsys) -> None:
+    result = emit_structured({"key": "val"}, as_json=False, as_yaml=True)
+    assert result is True
+    captured = capsys.readouterr()
+    parsed = yaml.safe_load(captured.out)
+    assert parsed["ok"] is True
+    assert parsed["data"]["key"] == "val"
+
+
+def test_emit_structured_returns_false_when_rich(monkeypatch) -> None:
+    monkeypatch.setenv("OUTPUT", "rich")
+    result = emit_structured({"key": "val"}, as_json=False, as_yaml=False)
+    assert result is False
+
+
+def test_emit_structured_wraps_already_wrapped_payload(capsys) -> None:
+    """If data is already in the agent schema, it should not be double-wrapped."""
+    already_wrapped = {"ok": True, "schema_version": "1", "data": "hello"}
+    emit_structured(already_wrapped, as_json=True, as_yaml=False)
+    captured = capsys.readouterr()
+    parsed = json.loads(captured.out)
+    assert parsed["data"] == "hello"
+    assert "data" not in str(parsed.get("data", {}) if isinstance(parsed.get("data"), dict) else "")