Add JIRA Groups API support

[Copilot]

Implements Groups API domain with search, CRUD operations, and membership management.

API Coverage

GroupDomainExtensions.cs:

• SearchAsync() - Query groups with exclusion filters
• CreateAsync() / DeleteAsync() - Group lifecycle with optional swap on delete
• AddUserAsync() / RemoveUserAsync() - Membership management (supports username/accountId)
• GetUsersAsync() - Paginated member retrieval

Entity Models:

• Group - Core group entity
• GroupWithUsers - Group with paginated user list

Integration:

• IGroupDomain interface following existing domain pattern
• JiraClient.Group property exposing domain

Usage

// Search for groups
var groups = await client.Group.SearchAsync("developers");

// Create and manage groups
var group = await client.Group.CreateAsync("api-team");
await client.Group.AddUserAsync("api-team", accountId: "user123");
var members = await client.Group.GetUsersAsync("api-team", maxResults: 50);

// Cleanup with visibility transfer
await client.Group.DeleteAsync("old-team", swapGroup: "new-team");

Implementation Notes

Uses type aliases (JiraGroup) to avoid conflict with System.Text.RegularExpressions.Group from global usings.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for JIRA Groups API</issue_title>
<issue_description>Add support for JIRA Groups API to:

• Search and find groups
• Create and delete groups
• Manage group membership
• Get users from groups

Implementation should be in a new GroupDomainExtensions.cs.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add support for JIRA Groups API #100

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