Inline suggestions (ghost text) never trigger for documents from third-party FileSystemProvider schemes (e.g. Code for IBM i "member:" scheme)

### Summary
GitHub Copilot's inline completion (ghost text) provider does not produce suggestions for documents opened through third-party FileSystemProvider implementations using a custom URI scheme — for example, the "member:" scheme used by the Code for IBM i extension (https://github.com/codefori/vscode-ibmi) to expose IBM i source members as editable documents.

### Environment
Copilot / Copilot Chat extension: latest. Code for IBM i extension: 3.0.13. OS: Windows.

### Steps to Reproduce
(1) Install the Code for IBM i extension and connect to an IBM i host.
(2) Open a source member for edit (e.g. member:/GCLIB/QRPGLESRC/DEMO02.SQLRPGLE?readonly=false).
(3) Confirm the editor is fully editable, and the language mode (bottom-right status bar) matches the file type (e.g. "RPGLE"), same as an equivalent local file.
(4) Start typing code that would normally trigger a ghost text suggestion.
(5) Compare with the same edit performed in a local file:// document of the same language.

### Expected Behavior
Inline suggestions (ghost text) appear for the member document, consistent with local files of the same language and content.

### Actual Behavior
No ghost text ever appears for the member: document, regardless of editor.inlineSuggest.enabled, language-specific overrides, or content. The Copilot status bar icon shows no warning/error state — identical to when Copilot is working normally. The "GitHub Copilot Chat" Output panel shows zero log/event activity when editing the member: document, whereas identical edits in a local file produce visible request/response log entries. This suggests the inline completion provider's document filter/selector excludes the member: scheme (and likely any non-standard scheme outside an internal allowlist such as file, untitled, vscode-remote, vscode-vfs, vsls) before a completion request is ever attempted — it is not a network, auth, or settings issue.

### Impact
Any extension that exposes remote/virtual content as editable text documents through a custom FileSystemProvider (Code for IBM i, and potentially other remote-development or legacy-system extensions) cannot offer Copilot ghost text to users editing that content, even though the documents are fully valid, editable TextDocuments with correct language IDs. Copilot Chat (chat panel / inline chat) is unaffected since it doesn't rely on the same provider registration.

### Request
Please consider one of the following: expanding the inline completion provider's scheme allowlist to include documents from any registered FileSystemProvider; providing a documented extension API / setting that lets third-party filesystem providers opt in to inline completion support; or, if this is intentional (e.g., for telemetry/content-exclusion reasons), documenting the supported scheme list so extension authors and users understand the limitation.

### Additional Context
Diagnosed by confirming inlineSuggest is enabled and works for local files; language mode and editability are identical between local and member documents; and the Copilot Chat Output panel shows zero activity for member documents but normal activity for local files, indicating the request is filtered out client-side before ever reaching Copilot's backend.