v2 `onCall` functions should provide configurable log level for invalid requests, not be hard-coded to `logger.error`

### Related issues

None

### [REQUIRED] Version info

**node:** v22.22.2

**firebase-functions:** v7.2.5

**firebase-tools:** v15.16.0

**firebase-admin:** v13.8.0

### [REQUIRED] Test case

If a Firebase v2 Callable function receives a `GET` request (instead of the correct `POST`), the Firebase Functions framework will log an error before it returns the 400 Bad Request response code

```
if (!isValidRequest(req)) {
logger.error("Invalid request, unable to process.");
throw new HttpsError("invalid-argument", "Bad Request");
}
```

https://github.com/firebase/firebase-functions/blob/master/src/common/providers/https.ts#L789

### [REQUIRED] Steps to reproduce

For any deployed Firebase function, if you get the URL of the function and trigger a `GET` request against it (eg due to misconfigured heartbeats, or even just a broken web crawler), you will then find an `ERROR` severity line in your Google Cloud Log Explorer, and an Open Error raised in your Google Cloud Error Reporting

### [REQUIRED] Expected behavior

For some use cases, the default of an `ERROR` severity to warn of a misconfiguration in your calling code is probably fine

However, for unauthenticated Firebase Functions which could be called by anyone (including web crawlers, AI bots, broken health checks etc), finding an `ERROR` severity log and a new Error Report every time someone somewhere on the internet tries to `GET` your function makes finding "real" issues hard amidst the noise.

Therefore, it would be better if the logging level for these invalid requests could be configured, so as the deployer of the function I can pick what severity level these invalid calls are recorded at.

### [REQUIRED] Actual behavior

My Google Cloud logs contain "not actually a real error but recorded as an error" lines such as:
```
2026-05-05 17:08:01.238
Error: Invalid request, unable to process.
at entryFromArgs (/workspace/node_modules/firebase-functions/lib/logger/index.js:144:19)
at Object.error (/workspace/node_modules/firebase-functions/lib/logger/index.js:131:11)
at /workspace/node_modules/firebase-functions/lib/common/providers/https.js:432:24
at /workspace/node_modules/firebase-functions/lib/common/providers/https.js:393:25
at cors (/workspace/node_modules/cors/lib/index.js:188:7)
at /workspace/node_modules/cors/lib/index.js:224:17
at originCallback (/workspace/node_modules/cors/lib/index.js:214:15)
at /workspace/node_modules/cors/lib/index.js:219:13
at optionsCallback (/workspace/node_modules/cors/lib/index.js:199:9)
at corsMiddleware (/workspace/node_modules/cors/lib/index.js:204:7)
```

### Were you able to successfully deploy your functions?

This only occurs when the Firebase HTTPS Callable Function is deployed and running successfully, and it receives a `GET` (or similar) non-POST request