fix: get_discussion graphQL invalid field

[LuluBeatson]

Fixes bug in get_discussion which caused tool error and returned:

Field 'state' doesn't exist on type 'Discussion'

• Removed "state" field from query and response
• Used github.Discussion object instead of github.Issue
• Corrected "labels" to "category" in response
• Tool arguments and descriptions unchanged.

Before with bug

Now fixed:

[Copilot]

Pull Request Overview

The PR fixes the GraphQL query and response handling in get_discussion by removing the invalid state field and replacing the old labels approach with a proper category mapping.

• Removed state from both query and response struct.
• Switched from github.Issue to github.Discussion return type and updated corresponding fields.
• Updated tests to reflect the new DiscussionCategory field instead of labels and state.

Reviewed Changes

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

File	Description
pkg/github/discussions.go	Removed State field, switched to github.Discussion, and mapped Category directly.
pkg/github/discussions_test.go	Updated test expectations: changed type to Discussion, removed State, and asserted on DiscussionCategory.

Comments suppressed due to low confidence (2)

pkg/github/discussions.go:215

• The function doc comment still refers to returning an issue. Please update any surrounding comments or docs to reflect that this now returns a Discussion struct rather than an Issue.

			discussion := &github.Discussion{

pkg/github/discussions_test.go:270

• [nitpick] Consider using a pointer (var out *github.Discussion) when unmarshalling JSON into a pointer type; this aligns with your expected pointer and avoids extra dereferencing in assertions.

			var out github.Discussion