docs: add multipart/form-data matching rule example

[JP-Ellis]

📝 Summary

Add an example which showcases how matching rules can be used to validate multipart/form-data payloads.

🚨 Breaking Changes

🔥 Motivation

The multipart/form-data payloads will typically contain additional structure internally. In order to validate this in Pact, this requires setting up matching rules, but in a situation where the typical 'embed the matcher with the value' doesn't work; instead requiring that the with_matching_rules method be called.

This documentation hopefully will help end-users validate these kinds of payloads.

🔨 Test Plan

Documentation is through an example, which is run as part of CI.

🔗 Related issues/PRs

• Fixes: How to combine with_multipart_file with additional body fields? #1069
• Fixes: Support mixed multipart/form‑data text fields and file parts in one interaction pact-reference#499

[Copilot]

Pull Request Overview

This PR adds documentation and examples for using Pact matching rules with multipart/form-data payloads. The examples demonstrate both consumer and provider tests for validating multipart requests containing JSON metadata and binary image data.

Key changes:

• Created a new patterns catalog structure with a multipart form-data example
• Added comprehensive documentation showing how to apply matching rules to individual parts within multipart bodies
• Included working test implementations for both consumer and provider sides

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
mkdocs.yml	Added Pydantic documentation inventory for cross-referencing
examples/catalog/pyproject.toml	New project configuration for the catalog examples with required dependencies
examples/catalog/multipart_matching_rules/test_provider.py	Provider verification test demonstrating FastAPI server handling multipart uploads
examples/catalog/multipart_matching_rules/test_consumer.py	Consumer test showing how to define matching rules for multipart request parts
examples/catalog/multipart_matching_rules/init.py	Empty init file for Python package
examples/catalog/multipart_matching_rules/README.md	Documentation explaining the multipart matching rules pattern
examples/catalog/init.py	Empty init file for catalog package
examples/catalog/README.md	Main catalog documentation with usage instructions
examples/README.md	Updated to reference the new patterns catalog

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51%. Comparing base (4dc54b3) to head (c494ce3).
⚠️ Report is 19 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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