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

[jvasnani]

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

[iloveeclipse]

Similar to #117

The reason and possible tool specific solution outlined in #117 (comment), more generic solution in #117 (comment)

You should provide adapter from Document to IFile. Once that is given, and LSP4E preference is set (see eclipse-lsp4e/lsp4e#1517), it should work.