Add get_work_item_types MCP tool for namespace type discovery

What does this MR do and why?

Adds a new MCP tool get_work_item_types that lets Duo agents discover the work item types available in a given namespace (group or project), including system-defined types (Issue, Epic, Task, …) and custom types (CWITs).

This is the first iteration of the work outlined in #592587.

Tool shape

Each entry in workItemTypes returns:

Field	Why
id	Global ID for use as workItemTypeId in workItemCreate
name	Human-readable type name (can be in any language / renamed)
iconName	For surfaces that render type chips
widgetDefinitions[].type	List of widget identifiers (e.g. ASSIGNEES, LABELS, HIERARCHY) so the agent knows which fields it can set on items of that type

Scope - intentionally minimal

Per the discussion in the issue (note 3173826951, note 3184213856), this MR ships the smallest useful surface that satisfies acceptance criteria 1 + 3 in the issue.

Deferred to follow-ups:

• Per-widget detail fields (allowsMultipleAssignees, allowedChildTypes/allowedParentTypes, custom field definitions). These bring in EE-only types and the ee_else_ce/work_item_type.fragment.graphql split — worth doing once we have a concrete agent need.
• Organization-level fallback for Self-Managed (organization { workItemTypes { ... } }).

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.

How to set up and validate locally

1. Enable the feature flags in the Rails console:

   Feature.enable(:mcp_client)
   Feature.enable(:mcp_catalog_agent_tools)

2. Create a copy of the Planner agent in your AI Catalog and enable it for your project.

3. Add the get work item types tool to the agent.

4. Create a custom work item type for your group.

5. Go to your group, open Duo chat, and select your agent.

6. Ask the agent to List available work item types.

7. The response should include all available work item types along with the custom work item type you created.

Related

Related issue: #592587

cc @acroitor @msaleiko @fredericcaplette @mmacfarlane - marking as Draft so we can align on scope before requesting review.