Add workspace file changes field definition and resolver to Workspace GraphQL type
MR: Pending
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## Description
As part of our [new designs for Workspaces](https://gitlab.com/groups/gitlab-org/-/epics/18674), we want the the frontend to display uncommitted file changes in expanded workspace cards to help users identify workspaces with unsaved work. To achieve this, we need to add a `changedFiles` field to the existing Workspace GraphQL type and implement its resolver.
## Acceptance criteria
- [ ] Add `fileChanges` field definition to existing Workspace GraphQL type
- [ ] Implement resolver method that returns information about locally modified files compared to the active branch
- [ ] Include change count and file paths for uncommitted changes
- [ ] Add GraphQL field documentation with examples
## TODO: Implementation plan
## Technical Notes
**Current Limitation:** As [noted by](https://gitlab.com/gitlab-org/gitlab/-/issues/563085#note_2702314222) @cwoolley-gitlab, we currently don't have the architecture to support this sort of visibility into the current state of the repo in a running workspace. We will need a way to sync workspace git status with Rails monolith.
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