docs: migrate test documentation from README_TESTS.md to CONTRIBUTING.rst

Move all test running instructions to CONTRIBUTING.rst under a new
'Running Tests' section with subsections for:
- Quick Start (Using Make)
- Manual Test Commands
- All Test Commands
- Using tox
- Continuous Integration

This consolidates test documentation in the standard CONTRIBUTING.rst
location where developers expect to find contribution guidelines.

Remove README_TESTS.md after content migration to avoid duplication.

Keep tests/conftest.py as it's required for nocov test mode where the
package is not installed in editable mode (usedevelop=false in tox.ini).

Author: jontsai

CONTRIBUTING.rst

@@ -73,8 +73,80 @@ For merging, you should:
 3. Add a note to ``CHANGELOG.rst`` about the changes.
 4. Add yourself to ``AUTHORS.rst``.
 
-Tips
-----
+Running Tests
+-------------
+
+Quick Start (Using Make)
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The easiest way to run tests::
+
+    # First time setup - create virtual environment
+    make venv
+    source .venv/bin/activate
+
+    # Install dependencies
+    make install
+
+    # Run all tests
+    make test
+
+    # Run just the catalog vendor tests
+    make test-vendor
+
+    # Run tests with coverage
+    make test-cov
+
+Manual Test Commands
+~~~~~~~~~~~~~~~~~~~~
+
+If you prefer to run pytest directly::
+
+    # Activate virtual environment
+    source .venv/bin/activate
+
+    # Run catalog extra fields tests
+    python -m pytest tests/test_vendor/test_catalog_v1.py -v
+
+All Test Commands
+~~~~~~~~~~~~~~~~~
+
+::
+
+    # Run all catalog vendor tests
+    python -m pytest tests/test_vendor/ -v
+
+    # Run specific test file
+    python -m pytest tests/test_vendor/test_catalog_v1.py -v
+
+    # Run specific test class
+    python -m pytest tests/test_vendor/test_catalog_v1.py::TestMetadataExtraFields -v
+
+    # Run specific test method
+    python -m pytest tests/test_vendor/test_catalog_v1.py::TestMetadataExtraFields::test_metadata_accepts_extra_fields -v
+
+    # Run with more verbose output
+    python -m pytest tests/test_vendor/test_catalog_v1.py -vv
+
+    # Run and show print statements
+    python -m pytest tests/test_vendor/test_catalog_v1.py -v -s
+
+    # Run all tests in the project
+    python -m pytest tests/ -v
+
+Using tox
+~~~~~~~~~
+
+The GitHub Actions CI uses tox to run tests across multiple Python and Pydantic versions::
+
+    # Run tests with Python 3.10 and Pydantic 2.10 (no coverage)
+    python3 -m tox -e py310-pydantic210-nocov
+
+    # Run tests with coverage
+    python3 -m tox -e py310-pydantic210-cover
+
+    # Run specific tests with tox
+    python3 -m tox -e py310-pydantic210-nocov -- tests/test_vendor/test_catalog_v1.py
 
 To run a subset of tests::
 
@@ -83,3 +155,14 @@ To run a subset of tests::
 To run all the test environments in *parallel*::
 
     tox -p auto
+
+Continuous Integration
+~~~~~~~~~~~~~~~~~~~~~~
+
+Tests run automatically on every push and pull request via GitHub Actions (``.github/workflows/github-actions.yml``).
+
+The CI runs tests across:
+
+* Python versions: 3.10, 3.11, 3.12, PyPy 3.9
+* Pydantic versions: 2.8, 2.10
+* With and without coverage reports