Gemini 3.8 Flash is missing from listModels() and silently falls back to Claude Sonnet 5

## Summary

With `@github/copilot-sdk@1.0.13`, the Copilot-hosted model `gemini-3.8-flash` is absent from `client.listModels()`. However, `client.createSession({ model: "gemini-3.8-flash" })` succeeds, and `session.rpc.model.getCurrent()` reports that Gemini is selected.

When a prompt is sent, the authoritative `assistant.usage` event reports that the request actually used `claude-sonnet-5`.

This results in three conflicting states:

```text
listed: false
selected: gemini-3.8-flash
used: claude-sonnet-5
```

There is no error or warning indicating that the requested model was unavailable or that fallback occurred.

## Environment

- `@github/copilot-sdk`: `1.0.13`
- Bundled Copilot CLI/runtime: `1.0.83`
- Node.js: `v22.17.0`
- npm: `11.6.2`
- OS: Windows NT `10.0.26200.0`
- Authentication: Explicit GitHub token for a Copilot-enabled user
- Provider: GitHub Copilot-hosted models, not BYOK
- Model: `gemini-3.8-flash`

This was reproduced with two separately authenticated GitHub accounts.

## Minimal reproduction

Install the current stable SDK:

```bash
npm install @github/copilot-sdk@1.0.13
```

Save the following as `repro.mjs`:

```js
import { CopilotClient } from "@github/copilot-sdk";

const requestedModel = "gemini-3.8-flash";
const gitHubToken = process.env.GITHUB_TOKEN ?? process.env.GH_TOKEN;

if (!gitHubToken) {
throw new Error("Set GITHUB_TOKEN or GH_TOKEN.");
}

const client = new CopilotClient({
gitHubToken,
useLoggedInUser: false,
});

let session;

try {
await client.start();

const models = await client.listModels();
console.log(
"listed:",
models.some(({ id }) => id === requestedModel),
);

session = await client.createSession({
model: requestedModel,
});

const current = await session.rpc.model.getCurrent();
console.log("selected:", current.modelId);

let actualModel;

session.on("assistant.usage", ({ data }) => {
actualModel = data.model;
});

const response = await session.sendAndWait(
"Reply with exactly OK.",
120_000,
);

console.log("used:", actualModel);
console.log("response:", response?.data.content);
} finally {
await session?.disconnect();
await client.stop();
}
```

Run it:

```bash
node repro.mjs
```

## Actual result

```text
listed: false
selected: gemini-3.8-flash
used: claude-sonnet-5
response: OK
```

The session-scoped model catalog also omits Gemini when queried with cache bypass:

```js
const result = await session.rpc.model.list({ skipCache: true });
```

## Expected result

One of the following would be consistent behavior:

1. If `gemini-3.8-flash` is available, it should be returned by the model-list APIs and used for the request.
2. If it is unavailable in the SDK/server-mode context, session creation or model switching should fail with a clear error.
3. If automatic fallback is intentional, the SDK should surface it explicitly, and `getCurrent()` should report the effective model rather than the unavailable requested model.

At minimum, `session.rpc.model.getCurrent()` and `assistant.usage.data.model` should not disagree without an accompanying model-change or fallback notification.

## Additional observations

- `client.listModels()` returned 21 models, with no Gemini models.
- `session.rpc.model.list({ skipCache: true })` returned 20 models, also with no Gemini models.
- The same behavior remained after explicitly enabling Gemini 3.8 Flash in the applicable Copilot model policy and waiting for the setting to propagate.
- Standalone Copilot CLI `1.0.83` can use `gemini-3.8-flash` successfully, and its usage data identifies Gemini as the actual model.
- This suggests a difference between standalone CLI startup model resolution and SDK/server-mode model discovery or dispatch.
- The issue is reproducible on a clean temporary Node.js project using the published SDK package.

Start with the provided repro.mjs, then trace client.listModels(), session.rpc.model.list({ skipCache: true }), session.rpc.model.getCurrent(), and the assistant.usage event. Compare SDK/server-mode model discovery and dispatch with the standalone CLI behavior; done means the requested, selected, and actually used models are consistent or fallback is explicitly reported.

索引モデルが issue の本文から書いたものです。