[Feat] Copilot Unable to Access Custom Virtual Files / Context in RCP Application

We are using a custom Eclipse RCP application that exposes a tree structure composed of virtual files. These files do not exist on the local filesystem until explicitly opened by the user.
Once opened, the files are materialized and stored in a predefined location within the user’s workspace.

However, the Copilot plugin is currently unable to:

• Access or index these virtual files
• Use them as context for suggestions
• Recognize files stored in the predefined workspace location

Observed Behavior

1. Copilot correctly reads and works with standard Eclipse project structures.
2. Copilot does not recognize or access:
   • Our custom tree structure (virtual file system)
   • Files stored in the predefined location (outside standard project structure)
3. Additionally:
   • Opened virtual files are not available in the "Add Context" menu
   • This appears to be because the files are not part of a recognized Eclipse project, even though they exist on disk after opening

Expected Behavior
Copilot should:

• Allow extending or configuring context sources
• Support custom file providers / virtual file systems
• Recognize files stored in non-standard but valid workspace locations
• Allow manually adding such files to context (e.g., via “Add Context”)

Key Limitation / Feature Gap
There is currently no way to configure or extend Copilot’s context resolution to:

• Include custom project structures
• Hook into virtual file systems
• Add files from arbitrary workspace locations

Suggested Enhancement
Provide a mechanism to:

• Register custom file providers or context resolvers
• Configure additional lookup locations for context
• Allow plugins (like RCP apps) to expose virtual files to Copilot

Start by reproducing the issue in a custom Eclipse RCP application, comparing standard project files with virtual files and opened files in the predefined workspace location. Trace the context resolution and “Add Context” entry points to determine where custom providers or locations could integrate. Done means virtual files can be indexed, selected as context, and used for suggestions without breaking standard Eclipse projects.