Enhance vCon party management and documentation

[howethomas]

• Updated the parties property to return a list of dictionaries instead of Party objects, allowing for in-place modifications.
• Introduced a new method, get_party_objects(), to retrieve Party objects from the internal data structure.
• Enhanced usage documentation to reflect changes in party data access and provide examples for both dictionary and object usage.
• Updated tests to align with the new party data structure and ensure correct assertions.

[Copilot]

Pull Request Overview

This PR enhances vCon party management by addressing a mutability inconsistency between the dialog and parties properties. The main purpose is to make both properties return mutable references that allow in-place modifications, ensuring consistent behavior across the API.

• Changed parties property to return dictionaries instead of Party objects for consistent mutability
• Added get_party_objects() method for backward compatibility when Party objects are needed
• Updated documentation and tests to reflect the new party data access patterns

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/vcon/vcon.py	Modified parties property to return mutable dictionaries and added get_party_objects() method
tests/test_vcon.py	Updated assertions to work with dictionary-based party access
tests/test_mutability_consistency.py	Added comprehensive tests demonstrating mutability consistency
docs/source/usage.rst	Enhanced documentation with examples for both dictionary and object usage
MUTABILITY_FIX_SUMMARY.md	Added detailed documentation of the changes and migration guide

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