test: add 105 new unit tests for untested core functionality

Add comprehensive tests for B-class issues (core functionality untested):

Pipeline:
- test_pool.py: QueryPool ID generation, caching, async context (12 tests)
- test_ratelimit.py: Fixed timing-sensitive test tolerance
- test_pipelinemgr.py: Use real Pydantic StageProcessResult instead of Mock

Utils:
- test_version.py: Version comparison functions (20 tests)
- test_logcache.py: Log page management and retrieval (18 tests)
- test_httpclient.py: HTTP session pool management (10 tests)
- test_proxy.py: Proxy configuration from env and config (10 tests)
- test_image.py: URL parsing and base64 extraction (12 tests)
- test_pkgmgr.py: Pip command generation (8 tests)

Discover:
- test_engine.py: I18nString, Metadata, Component manifest (15 tests)

Test count: 1193 → 1298 (+105 tests)

Note: Some B-class issues cannot be tested due to circular import bugs
filed as GitHub issues #2175 (pipeline) and #2176 (persistence).

tests/unit_tests/utils/test_pkgmgr.py

@@ -0,0 +1,102 @@
+"""
+Unit tests for package manager utilities.
+
+Tests pip command generation without actual installation.
+"""
+
+from __future__ import annotations
+
+import pytest
+from unittest.mock import patch, Mock
+
+from langbot.pkg.utils import pkgmgr
+
+
+class TestPkgMgr:
+    """Tests for package manager functions."""
+
+    def test_install_calls_pipmain(self):
+        """install calls pipmain with correct arguments."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.install('requests')
+
+            mock_pipmain.assert_called_once_with(['install', 'requests'])
+
+    def test_install_with_version(self):
+        """install handles package with version specifier."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.install('requests>=2.0.0')
+
+            mock_pipmain.assert_called_once_with(['install', 'requests>=2.0.0'])
+
+    def test_install_upgrade_calls_pipmain(self):
+        """install_upgrade calls pipmain with upgrade and mirror."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.install_upgrade('requests')
+
+            expected_args = [
+                'install',
+                '--upgrade',
+                'requests',
+                '-i',
+                'https://pypi.tuna.tsinghua.edu.cn/simple',
+                '--trusted-host',
+                'pypi.tuna.tsinghua.edu.cn',
+            ]
+            mock_pipmain.assert_called_once_with(expected_args)
+
+    def test_run_pip_with_params(self):
+        """run_pip passes params to pipmain."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.run_pip(['list', '--outdated'])
+
+            mock_pipmain.assert_called_once_with(['list', '--outdated'])
+
+    def test_run_pip_empty_params(self):
+        """run_pip handles empty params."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.run_pip([])
+
+            mock_pipmain.assert_called_once_with([])
+
+    def test_install_requirements_calls_pipmain(self):
+        """install_requirements calls pipmain with requirements file."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.install_requirements('requirements.txt')
+
+            expected_args = [
+                'install',
+                '-r',
+                'requirements.txt',
+                '-i',
+                'https://pypi.tuna.tsinghua.edu.cn/simple',
+                '--trusted-host',
+                'pypi.tuna.tsinghua.edu.cn',
+            ]
+            mock_pipmain.assert_called_once_with(expected_args)
+
+    def test_install_requirements_with_extra_params(self):
+        """install_requirements handles extra params."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.install_requirements('requirements.txt', ['--no-cache-dir'])
+
+            expected_args = [
+                'install',
+                '-r',
+                'requirements.txt',
+                '-i',
+                'https://pypi.tuna.tsinghua.edu.cn/simple',
+                '--trusted-host',
+                'pypi.tuna.tsinghua.edu.cn',
+                '--no-cache-dir',
+            ]
+            mock_pipmain.assert_called_once_with(expected_args)
+
+    def test_install_requirements_multiple_extra_params(self):
+        """install_requirements handles multiple extra params."""
+        with patch('langbot.pkg.utils.pkgmgr.pipmain') as mock_pipmain:
+            pkgmgr.install_requirements('requirements.txt', ['--no-cache-dir', '--verbose'])
+
+            call_args = mock_pipmain.call_args[0][0]
+            assert '--no-cache-dir' in call_args
+            assert '--verbose' in call_args