[test] Add code coverage framework

Implement a comprehensive test coverage framework that integrates with pytest-cov
to generate code coverage reports for the yt-dlp codebase. Key components:

- Configuration file (.coveragerc) with appropriate include/exclude patterns
- Helper script (run_coverage.py) with parallel report generation
- GitHub Actions workflow for automatic coverage reporting on PRs and pushes
- Support for Hatch testing environment and CLI integration
- Testing documentation for running coverage reports
- Sample test for demonstrating coverage reporting

devscripts/cov-combine

@@ -0,0 +1,5 @@
+#!/bin/sh
+# This script is a helper for the Hatch test coverage command
+# It's called by `hatch test --cover`
+
+coverage combine "$@"

devscripts/run_coverage.py

@@ -0,0 +1,105 @@
+#!/usr/bin/env python3
+
+# Script to run coverage tests for yt-dlp
+#
+# Usage:
+#   python -m devscripts.run_coverage [test_path] [module_path] [additional pytest args]
+#
+# Examples:
+#   python -m devscripts.run_coverage                         # Test everything
+#   python -m devscripts.run_coverage test/devscripts         # Test devscripts
+#   python -m devscripts.run_coverage test/test_utils.py yt_dlp.utils  # Test specific module
+#   python -m devscripts.run_coverage test/test_utils.py "yt_dlp.utils,yt_dlp.YoutubeDL"  # Test multiple modules
+#   python -m devscripts.run_coverage test -v                 # With verbosity
+#
+# Using hatch:
+#   hatch run hatch-test:run-cov [args]                       # Same arguments as above
+#   hatch test --cover                                        # Run all tests with coverage
+#
+# Important:
+#   - Always run this script from the project root directory
+#   - Test paths are relative to the project root
+#   - Module paths use Python import syntax (with dots)
+#   - Coverage reports are generated in .coverage-reports/
+
+import sys
+import subprocess
+from pathlib import Path
+
+script_dir = Path(__file__).parent
+repo_root = script_dir.parent
+
+
+def main():
+    args = sys.argv[1:]
+
+    if not args:
+        # Default to running all tests
+        test_path = 'test'
+        module_path = 'yt_dlp,devscripts'
+    elif len(args) == 1:
+        test_path = args[0]
+        # Try to guess the module path from the test path
+        if test_path.startswith('test/devscripts'):
+            module_path = 'devscripts'
+        elif test_path.startswith('test/'):
+            module_path = 'yt_dlp'
+        else:
+            module_path = 'yt_dlp,devscripts'
+    else:
+        test_path = args[0]
+        module_path = args[1]
+
+    # Initialize coverage reports directory
+    cov_dir = repo_root / '.coverage-reports'
+    cov_dir.mkdir(exist_ok=True)
+    html_dir = cov_dir / 'html'
+    html_dir.mkdir(exist_ok=True)
+
+    # Run pytest with coverage
+    cmd = [
+        'python', '-m', 'pytest',
+        f'--cov={module_path}',
+        '--cov-config=.coveragerc',
+        '--cov-report=term-missing',
+        test_path,
+    ]
+
+    if len(args) > 2:
+        cmd.extend(args[2:])
+
+    print(f'Running coverage on {test_path} for module(s) {module_path}')
+    print(f'Command: {" ".join(cmd)}')
+
+    try:
+        result = subprocess.run(cmd, check=True)
+
+        # Generate reports after the test run in parallel
+        import concurrent.futures
+
+        def generate_html_report():
+            return subprocess.run([
+                'python', '-m', 'coverage', 'html',
+            ], check=True)
+
+        def generate_xml_report():
+            return subprocess.run([
+                'python', '-m', 'coverage', 'xml',
+            ], check=True)
+
+        with concurrent.futures.ThreadPoolExecutor(max_workers=2) as executor:
+            html_future = executor.submit(generate_html_report)
+            xml_future = executor.submit(generate_xml_report)
+            # Wait for both tasks to complete
+            concurrent.futures.wait([html_future, xml_future])
+
+        print(f'\nCoverage reports saved to {cov_dir.as_posix()}')
+        print(f'HTML report: open {cov_dir.as_posix()}/html/index.html')
+        return result.returncode
+    except subprocess.CalledProcessError as e:
+        print(f'Error running coverage: {e}')
+        return e.returncode
+
+
+if __name__ == '__main__':
+    sys.exit(main())