Uses a representative sample of related cases for the GenAI analysis

[mvilanova]

This pull request includes several important changes to the dispatch_slack and signal services to enhance functionality and improve code documentation. The most important changes include adding a new method to fetch cases by resolution reason, updating imports, and adding docstrings for better code clarity.

Enhancements to case fetching and signal analysis:

• src/dispatch/plugins/dispatch_slack/case/messages.py: Added a new method to fetch cases based on resolution reason in the create_genai_signal_analysis_message function.
• src/dispatch/signal/service.py: Added a new method get_cases_for_signal_by_resolution_reason to retrieve cases associated with a given signal and resolution reason.

Code documentation improvements:

• src/dispatch/signal/service.py: Added docstrings for the get_instances_in_case and get_cases_for_signal methods to improve code readability and maintainability. [1] [2]

Import updates:

• src/dispatch/plugins/dispatch_slack/case/messages.py: Updated imports to include CaseResolutionReason from dispatch.case.enums.