feat: add webhook tools for projects and groups

[DarkByteZero]

Summary

• Adds 3 new unified webhook tools: list_webhooks, list_webhook_events, get_webhook_event
• Works for both project and group scoped webhooks via project_id or group_id parameter
• New webhooks toolset (opt-in, isDefault: false)

Tools

Tool	Description
list_webhooks	List configured webhooks for a project or group
list_webhook_events	List recent events (7 days) with summary mode and status filter
get_webhook_event	Get full payload of a specific event by ID with optional page hint

Key Features

• Summary mode: list_webhook_events with summary: true returns only id, url, trigger, response_status, execution_duration to avoid huge payloads
• Status filter: Filter events by status code (200, 500) or category (successful, client_failure, server_failure)
• Event lookup: get_webhook_event supports an optional page parameter for direct single-call lookup, or auto-paginates up to 25 pages (500 events) when page is unknown
• Pagination: GitLab enforces max per_page=20 for webhook events, all tools respect this limit

GitLab API endpoints used

• GET /projects/:id/hooks / GET /groups/:id/hooks
• GET /projects/:id/hooks/:hook_id/events / GET /groups/:id/hooks/:hook_id/events

Test plan

• Verify list_webhooks returns webhooks for a project
• Verify list_webhooks returns webhooks for a group
• Verify list_webhook_events with summary: true returns compact results
• Verify list_webhook_events with status filter works
• Verify get_webhook_event finds a specific event by ID
• Verify get_webhook_event with page hint does single API call
• Verify error when neither project_id nor group_id is provided
• Verify toolset filtering works (GITLAB_TOOLSETS=webhooks)