Enterprise models omitted from headless model catalogs; switching fails with "Provider model is not registered" #5328

Describe the bug

Enterprise-managed models appear in the authenticated Copilot API /models response with model_picker_enabled: true, but headless runtime discovery omits all enterprise models available to the tested account.

This affects SDK client.listModels(), native client.rpc.models.list({ gitHubToken }), and session model.list(). Comparing the API and native RPC with the same GitHub credential reproduces the omission.

Calling session.setModel() with an omitted enterprise model's exact catalog ID also fails:

Request session.model.switchTo failed with message:
Provider model '<enterprise>/<provider>/<model>' is not registered

Affected version

• CLI/runtime 1.0.105, SDK 1.0.28 — reproduced with both the bundled runtime and installed CLI.
• Bundled runtime 1.0.106-2, SDK 1.0.29-preview.1 — also reproduces.
• Originally observed with SDK 1.0.13 and CLI 1.0.105.

Steps to reproduce the behavior

1. Authenticate an account entitled to an enterprise-managed model.
2. Confirm its exact ID appears in GET https://api.githubcopilot.com/models with model_picker_enabled: true.
3. Start an SDK client over stdio with the bundled runtime.
4. Compare client.listModels() and client.rpc.models.list({ gitHubToken }), using the same credential as the API request. Enterprise models are absent.
5. Create a temporary session and call session.rpc.model.list(). Enterprise models are also absent.
6. Call session.setModel() with the exact enterprise catalog ID. Observe the registration error above.

Expected behavior

Entitled enterprise-managed models should be discoverable and selectable through the headless runtime, or an explicit limitation should be documented.

Additional context

• macOS, arm64, zsh.
• Reproduced with fresh clients and temporary sessions. No inference prompts were sent.
• Creating a session with the enterprise model ID records that ID, but successful inference has not been verified.
• Enterprise identifiers are redacted; the error above uses placeholders.

Related: #4960 (marked fixed in CLI 1.0.95), #4953 (encoded-slash selection), and #1608 (incomplete RPC catalog). This reproduction persists on newer stable and preview runtimes; a shared root cause is not confirmed.