[GenAI Prompt API] 1.0.0-beta3 regression: default ModelConfig switched the requested AICore feature from 636 to 648, which no shipping AICore provides

### Summary

`com.google.mlkit:genai-prompt:1.0.0-beta3` cannot reach the Prompt API at all on a Pixel 10 where **beta1 and beta2 work fine**. `checkStatus()` throws `GenAiException(errorCode=606) FEATURE_NOT_FOUND` for every `ModelConfig`.

The cause is a **change in the requested AICore feature ID**: the default `ModelConfig` requests feature **636** in beta2 and feature **648** in beta3. No shipping AICore provides 648.

I verified this is **not** an AICore configuration-download issue: **beta2 succeeds on the same device, in the same session, minutes apart, with the same source code.** Only the AAR version differs.

### Environment

| | |
|---|---|
| Device | Google Pixel 10 (`frankel`), Tensor G5 |
| OS | Android 16, build `CP1A.260505.005`, security patch 2026-05-05 |
| Bootloader | **locked** (`ro.boot.verifiedbootstate=green`, `ro.boot.flash.locked=1`) |
| AICore | `0.release.prod_aicore_20260528.01_RC08.934495584` (versionCode 456989), **release channel**, installed from Play, no update offered |
| Play services | 26.23.34 |
| Google Play system update | 2026-07-01S+ |
| Account | not a member of `aicore-experimental` or any AICore testing group |

Pixel 10 is listed as a supported nano-v3 device for the Prompt API on the [official device support page](https://developers.google.com/ml-kit/genai#prompt-device).

### Steps to reproduce

1. `Generation.getClient()` (or `getClient(GenerationConfig)` with any `ModelConfig`)
2. `checkStatus()`
3. It throws instead of returning a `FeatureStatus`.

```
GenAiException(errorCode=606): [ErrorCode 606] AICore failed with error type
3-PREPARATION_ERROR and error code 606-FEATURE_NOT_FOUND: Feature 648 is not available.
```

### Evidence 1 — version A/B/A on one device, one session

Same source code, same device, same AICore. Only the `genai-prompt` version changed.

| Time | genai-prompt | `checkStatus()` (default `ModelConfig`) |
|---|---|---|
| 10:38 | **beta3** | throws 606, Feature **648** not available |
| 10:54 | **beta2** | returns **3 (AVAILABLE)** — inference then runs normally |
| 10:56 | **beta3** | throws 606, Feature **648** not available |
| 11:28 | **beta1** | returns **3 (AVAILABLE)** — inference then runs normally |

**This rules out "AICore hasn't finished downloading the latest configurations"** (the documented cause of 606): if the config were missing, beta1/beta2 would fail too. The device had also been rebooted and left connected to Wi-Fi, per the [troubleshooting guidance](https://developers.google.com/ml-kit/genai/prompt/android/get-started).

Additional control: **ML Kit GenAI Summarization works on the same device in the same session** (`nano-v3`, ~9.9 s for a Japanese article). AICore itself is healthy.

### Evidence 2 — the requested feature ID changed from 636 to 648

On the Pixel 10, beta2 **succeeds**, so its requested feature ID is not visible (the ID only appears in the error message).

To observe it, I ran the **same beta2 build on a Pixel 9a**, where the Prompt API is not provisioned at all. There, beta2 fails and prints the ID it asked for:

| `ModelConfig` | beta2 requests | beta3 requests |
|---|---|---|
| `default` | **636** | **648** |
| `STABLE` × `FULL` | **636** | **648** |
| `STABLE` × `FAST` | 645 | 645 |
| `PREVIEW` × `FAST` | 646 | 646 |
| `PREVIEW` × `FULL` | 647 | 647 |

Feature IDs are requested by the library, not reported by the device, so this is a property of the AAR version.

**beta3 changed `default` / `STABLE`×`FULL` from 636 to 648.** 636 exists on the shipping AICore (`prod_aicore_20260528`); 648 does not. Every publicly reported feature ID for beta1/beta2 that I could find is also 636, which is consistent.

In beta3, **no `ModelConfig` combination is reachable** — there is no workaround at the API level. (646/647 being absent is expected: PREVIEW requires the AICore Developer Preview program. The problem is the STABLE path.)

### Evidence 3 — the regression is in `genai-prompt`, not `genai-common`

beta3 also bumps its `genai-common` dependency from beta3 to beta4. To separate the two, I forced `genai-common:1.0.0-beta4` while keeping `genai-prompt:1.0.0-beta2`:

| Combination | Result |
|---|---|
| genai-prompt beta2 + genai-common beta3 (default) | works |
| **genai-prompt beta2 + genai-common beta4 (forced)** | **works** |
| genai-prompt beta3 + genai-common beta4 (default) | fails |

So the regression is in **genai-prompt beta3 itself**.

### Expected

`genai-prompt:1.0.0-beta3` should request an AICore feature that shipping AICore builds actually provide — or the release notes should state the minimum AICore version it requires. Currently [the release notes](https://developers.google.com/ml-kit/release-notes) list beta3 with a date and no description, and the get-started page still documents beta2.

Start at the Android Prompt API entry points named in the report, Generation.getClient(), ModelConfig, and checkStatus(), and compare genai-prompt beta2 with beta3. Reproduce the feature ID difference on the stated devices and verify that the fix makes beta3 use a feature supplied by shipping AICore, or documents the required minimum AICore version.

Written by the indexing model from the issue text.