Params CompareExpression `.thenElse` does not support `RegExp` as value, therefore disallows the use of RegExp values for CORS options

### Related issues

#1787
#1406
#1903

Actually I am trying to do this https://github.com/firebase/firebase-functions/pull/1903#issuecomment-5105925317 - however it turned out that #1903 unfortunately does not support setting regex CORS options directly from the .env files.

### [REQUIRED] Version info

**node:** v24.18.0

**firebase-functions:** v7.3.0

**firebase-tools:** v15.23.0

**firebase-admin:** v13.10.0

### [REQUIRED] Test case

```typescript
const environmentVariable_Some = defineBoolean('SOME')
const environmentVariable_Staging = defineBoolean('IS_STAGING')

const corsOptionsLocal = [/^http:\/\/localhost:808[0|1]$/]
const corsOptionsStaging = [/^https:\/\/(staging|otherstaging)\.example\.com$/]
const corsOptionsProduction = [/^https:\/\/(some|other)\.example\.com$/]

const corsOptions = environmentVariable_Some
.equals(true)
.thenElse(corsOptionsLocal, environmentVariable_Staging.equals(true).thenElse(corsOptionsStaging, corsOptionsProduction))

export const myFn = onCall(
{
cors: corsOptions
},
async () => {
...
}
```

### [REQUIRED] Steps to reproduce

1. Run `npx tsc`

### [REQUIRED] Expected behavior

I can set dynamic (RegExp-based) allowed CORS hosts dynamically depending on the environment.
The `onCall` `cors` option allows the following types:
```
(property) HttpsOptions.cors?: string | boolean | RegExp | Expression | Expression | (string | RegExp)[] | undefined
```
...so it should also be possible to create a `Expression` using the `.equals(xx).thenElse(...)` `CompareExpression` helper.

### [REQUIRED] Actual behavior

```
Argument of type 'RegExp[]' is not assignable to parameter of type 'string | number | boolean | string[] | Expression'.
Type 'RegExp[]' is not assignable to type 'string[]'.
Type 'RegExp' is not assignable to type 'string'.
```

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

No