Filter work item types by custom work item type name and onlyAvailable
## Problem
Following the implementation in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/224836, the `WorkItems::TypesFinder` now returns both system-defined and custom work item types via the Provider.
Since these queries like `organization.workItemTypes` or `namespace.workItemTypes` also accept the `name` argument and `onlyAvailable` arguments, we need to ensure that custom work item types are also supported in filtering.
## Current State
* ✅ Provider returns system-defined + custom types when `work_item_configurable_types` feature flag is enabled
* ✅ Provider falls back to system-defined types only when feature flag is disabled (backward compatibility)
* ⚠️ Filtering by `name` and `onlyAvailable` only works with system-defined types
* ⚠️ Custom work item types are not properly handled in filtering scenarios
## Affected Queries
This change affects the following GraphQL queries that retrieve work item types:
* `organization.workItemTypes`
* `namespace.workItemTypes`
* `group.workItemTypes`
* `project.workItemTypes`
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## Proposal
Both system defined and custom work item types can be filtered by `name` (via `workItemTypeIds`) and `onlyAvailable` arguments.
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