Background task agents silently substitute requested model without warning

### Describe the bug

When launching a background agent via the task tool with an explicit model parameter, the agent silently substitutes
a different model and completes without any indication of the substitution — until you inspect the model: field in
the result metadata.

Environment: GitHub Copilot CLI v1.0.44, Windows

### Affected version

GitHub Copilot CLI 1.0.44

### Steps to reproduce the behavior

Steps to reproduce:

1. Launch a background agent with model: "claude-opus-4.7" or model: "gpt-5.5" or model: "gpt-5.3-codex", by prompting "do a tri-model review using claude opus 4.7, gpt 5.5 and gpt 5.3 codex" with claude sonnet as the active model.
2. Agent completes with status: idle
3. Read result — model: claude-sonnet-4.6 appears in the metadata, not the requested model

Actual behaviour:

Agent silently downgrades to claude-sonnet-4.6 and returns a result. No warning, no error, nothing in the output
indicating the substitution occurred. The substitution is only discoverable by reading the raw metadata line model:
in the agent result.

Why this matters:

Workflows that depend on specific model capabilities (e.g., multi-model review loops requiring claude-opus-4.7,
gpt-5.5, and gpt-5.3-codex for independent perspectives) are silently invalidated. The caller believes they got
three independent reviews when in fact two were the same model. gpt-5.3-codex appeared to work correctly; it was
claude-opus-4.7 and gpt-5.5 that substituted.

### Expected behavior

Either:

- Use the requested model, or
- Fail with a clear error: "Model 'claude-opus-4.7' is not available in this environment" so the caller can decide
how to proceed

### Additional context

_No response_

Start by reproducing the background task with an explicit model such as claude-opus-4.7 or gpt-5.5, then inspect the result metadata where the substituted model is shown. Trace the task tool's model selection and make completion either use the requested model or report that it is unavailable; verify that silent substitution no longer occurs.

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