Add missing enum constraints

[tonytrg]

This pr adds some missing enum constraints for string props with enum like categories.
It also removes those listed enums from the descriptions, as the llm already receives the list of possible candidates by the the enum constraint.

This will save tokens as noted by @SamMorrowDrums

In regard to #228

Example testing with Copilot(Model Claude 3.7 Sonnet):

Please list me all code scanning alerts from:
owner tonytrg
repo adventofcode
severity ultra
GitHub Copilot
I need to help you retrieve code scanning alerts from the GitHub repository, but I notice that you've specified a severity level of "ultra" which isn't a standard severity level for GitHub code scanning alerts.
GitHub's code scanning alerts use these severity levels: critical, high, medium, low, warning, note, and error. The value "ultra" isn't one of the supported severity options.
Would you like me to list the alerts with a different severity level? For example, would you prefer "critical" (highest severity), "high", or another valid severity level?

[Copilot]

Pull Request Overview

This PR adds missing enum constraints for various string properties to ensure only the allowed values are accepted.

• Enforced enum constraints for pull request state, sort options, merge methods, and review event types in pkg/github/pullrequests.go.
• Added enum constraints for code scanning alert states and severity levels in pkg/github/code_scanning.go.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/github/pullrequests.go	Added enum constraints for pull request query and review parameters.
pkg/github/code_scanning.go	Added enum constraints for code scanning alerts parameters.

[SamMorrowDrums]

Added one minor suggestion (the word category doesn't add much I think) but it's a nit, and this looks great. Thanks for all your work on finding all these!