[app_check]: iOS build fails with "Cannot find 'RecaptchaProvider' in scope" and "Redundant conformance of 'FlutterError'"

### Which plugins are affected?

App Check

### Which platforms are affected?

iOS, macOS (any platform where the "recaptcha" Apple/Web provider switch case in `FirebaseAppCheckPlugin.swift` is compiled — this fails at compile time regardless of which provider is actually activated at runtime)

### Description

`firebase_app_check` 0.4.6 and 0.4.7 fail to build on iOS with two Swift compiler errors, unrelated to which App Check provider the app actually activates (we only use `AppleDebugProvider`, never reCAPTCHA):

**1. `Cannot find 'RecaptchaProvider' in scope`**

Introduced in [PR #18505](https://github.com/firebase/flutterfire/pull/18505) ("rCE provider api update", merged 2026-08-03), `FirebaseAppCheckPlugin.swift` calls:

```swift
delegateProvider = RecaptchaProvider(app: app, siteKey: recaptchaSiteKey)
```

`AppCheckCore` (resolved at 11.3.1 in our lockfile) does not expose a class named `RecaptchaProvider`. The actual class is `AppCheckRecaptchaProvider`, declared in `AppCheckCore/AppCheckRecaptchaProvider/Sources/Public/AppCheckRecaptchaProvider.swift`:

```swift
public final class AppCheckRecaptchaProvider: NSObject, AppCheckCoreProvider {
```

Renaming the call site to `AppCheckRecaptchaProvider(app: app, siteKey: recaptchaSiteKey)` does not fix it either — `AppCheckCore` is not implicitly imported by `FirebaseAppCheckPlugin.swift` (only `FirebaseAppCheck` and `FirebaseCore` are), so the type isn't in scope at all until `import AppCheckCore` is added. Even after adding that import, the constructor signature and protocol conformance have changed too: `AppCheckRecaptchaProvider` conforms to `AppCheckCoreProvider`, not `AppCheckProvider`, and its initializer takes `resourceName`/`APIKey` parameters, not `app`/`siteKey`. So this isn't a simple rename — the "recaptcha" case in `configure(app:providerName:debugToken:recaptchaSiteKey:)` needs a real rewrite against the current `AppCheckRecaptchaProvider` API.

**2. `Redundant conformance of 'FlutterError' to protocol 'Error'`**

`FirebaseAppCheckPlugin.swift` declares:

```swift
// swift-format-ignore: AvoidRetroactiveConformances
extension FlutterError: @retroactive Error {}
```

Several other Firebase Flutter plugins already linked into a typical app declare this exact same extension in their own Swift plugin file (confirmed present in `cloud_functions`, `firebase_storage`, `firebase_remote_config`, `firebase_analytics` — at both their currently-latest versions and several versions back). Any app that also depends on one of those (which is effectively every non-trivial FlutterFire app) gets a "redundant conformance" compile error as soon as `firebase_app_check` is added, independent of the `firebase_core` version in use — we reproduced it with `firebase_core` pinned all the way back to 4.13.0 (the last release before its own Objective-C→Swift rewrite in 4.14.0), so it isn't specific to that rewrite either.

### Reproducing the issue

1. Create a fresh Flutter app (or use an existing one) that already depends on at least one other Firebase plugin whose Swift implementation declares `extension FlutterError: @retroactive Error {}` — e.g. `cloud_functions`, `firebase_storage`, `firebase_remote_config`, or `firebase_analytics`.
2. Add `firebase_app_check: ^0.4.6` (or `^0.4.7`).
3. `flutter pub get`, then `cd ios && rm -rf Pods Podfile.lock && pod install` (or just `flutter run -d `), targeting iOS.
4. Build fails with both errors above, regardless of whether `FirebaseAppCheck.instance.activate(...)` is ever called with a debug/device-check/app-attest provider — the file fails to *compile*, not just to run.

We did not need to actually invoke the "recaptcha" provider at runtime to hit this — Swift has to type-check the whole switch statement in `configure(...)` regardless of which case executes.

### Firebase Core version

4.14.0 (also reproduced with 4.13.0, see above)

### Flutter Version

3.44.1 (stable)

### Relevant Log Output

```
Swift Compiler Error (Xcode): Redundant conformance of 'FlutterError' to protocol 'Error'
.../firebase_app_check-0.4.6/ios/firebase_app_check/Sources/firebase_app_check/FirebaseAppCheckPlugin.swift:23:37

Swift Compiler Error (Xcode): Cannot find 'RecaptchaProvider' in scope
.../firebase_app_check-0.4.6/ios/firebase_app_check/Sources/firebase_app_check/FirebaseAppCheckPlugin.swift:381:29

Could not build the application for the simulator.
```

### Flutter dependencies

Expand Flutter dependencies snippet

```yaml
firebase_app_check 0.4.6
firebase_core 4.14.0
cloud_functions 6.4.0
firebase_storage 13.5.0
firebase_remote_config 6.6.0
firebase_auth 6.6.1
```

### Additional context and comments

This blocks any app using `firebase_app_check` on iOS alongside `cloud_functions`/`firebase_storage`/`firebase_remote_config`/`firebase_analytics`, which is likely a very common combination.

Possible fix direction for the "recaptcha" case: update it to construct `AppCheckRecaptchaProvider` with its current initializer (`resourceName`/`APIKey`, conforming to `AppCheckCoreProvider` rather than `AppCheckProvider`) and add `import AppCheckCore` to `FirebaseAppCheckPlugin.swift`, since it isn't currently re-exported through `FirebaseAppCheck`. For the duplicate `FlutterError` conformance, since several sibling plugins already declare it, `firebase_app_check` likely doesn't need its own copy at all.

Start in ios/firebase_app_check/Sources/firebase_app_check/FirebaseAppCheckPlugin.swift and compare its reCAPTCHA case with AppCheckCore/AppCheckRecaptchaProvider/Sources/Public/AppCheckRecaptchaProvider.swift. Reproduce the iOS build with another plugin that declares the FlutterError conformance, then verify the current App Check provider API and duplicate-conformance behavior. Done means the listed dependencies compile without either Swift error.

Written by the indexing model from the issue text.