Firebase Auth with Microsoft (Entra AD) - Login not possible - "Unable to process request due to missing initial state"

### Description

Environment: Flutter
Android Studio: 2026.1.1 Patch 1
firebase_core: 4.10.0
Firebase iOS SDK version: 12.14.0
Installation method: SPM
Firebase Component: Auth

When logging in to Firebase with Microsofts Entra AD as an authentication service by using `FirebaseAuth.instance.signInWithProvider(microsoftProvider)` I am able to open the Microsoft login page.
After entering the correct email + password then the following error appears inside the In-App-Browser:
"_Unable to process request due to missing initial state. This may happen if browser sessionStorage is inaccessible or accidentally cleared. Some specific scenarios are - 1) Using IDP-Initiated SAML SSO. 2) Using signInWithRedirect in a storage-partitioned browser environment._"

The second login-try afterwards always works.
Most likely because the session has a valid Microsoft token after the first try already, so no redirect is necessary anymore.

We already had this topic in #7153 natively in Swift, but it seems to appear more often again, but don't ask me why.
Maybe changes in one of you last Firebase SDK versions?

### Reproducing the issue

The problem appears no matter if I set Chrome or Safari as a standard browser in iOS settings.
It happens on real devices (iPhone and iPad, iOS 26.5.1 and others), but not every user is affected. Most users can login without any problems.
I can't determine in what kind of circumstances the problem occurs, but on a device where the problem occurs, it is always reproducible.

### Firebase SDK Version

12.14.0

### Xcode Version

26.5

### Installation Method

Swift Package Manager

### Firebase Product(s)

Authentication

### Targeted Platforms

iOS

### Relevant Log Output

```shell
No log output in this context.
The error message appears within a browser frame.
```

### If using Swift Package Manager, the project's Package.resolved

Expand Package.resolved snippet

```json

{
"pins" : [
{
"identity" : "abseil-cpp-binary",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/abseil-cpp-binary.git",
"state" : {
"revision" : "bbe8b69694d7873315fd3a4ad41efe043e1c07c5",
"version" : "1.2024072200.0"
}
},
{
"identity" : "app-check",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/app-check.git",
"state" : {
"revision" : "61b85103a1aeed8218f17c794687781505fbbef5",
"version" : "11.2.0"
}
},
{
"identity" : "firebase-ios-sdk",
"kind" : "remoteSourceControl",
"location" : "https://github.com/firebase/firebase-ios-sdk",
"state" : {
"revision" : "8d5b4189f1f482df8d5c58c9985ea70491ef5382",
"version" : "12.14.0"
}
},
{
"identity" : "google-ads-on-device-conversion-ios-sdk",
"kind" : "remoteSourceControl",
"location" : "https://github.com/googleads/google-ads-on-device-conversion-ios-sdk",
"state" : {
"revision" : "9bfcc6cf435b2e7c5562c1900b8680c594fa9a64",
"version" : "3.6.0"
}
},
{
"identity" : "googleappmeasurement",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/GoogleAppMeasurement.git",
"state" : {
"revision" : "219e564a8510e983e675c94f77f7f7c50049f22d",
"version" : "12.14.0"
}
},
{
"identity" : "googledatatransport",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/GoogleDataTransport.git",
"state" : {
"revision" : "617af071af9aa1d6a091d59a202910ac482128f9",
"version" : "10.1.0"
}
},
{
"identity" : "googleutilities",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/GoogleUtilities.git",
"state" : {
"revision" : "60da361632d0de02786f709bdc0c4df340f7613e",
"version" : "8.1.0"
}
},
{
"identity" : "grpc-binary",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/grpc-binary.git",
"state" : {
"revision" : "75b31c842f664a0f46a2e590a570e370249fd8f6",
"version" : "1.69.1"
}
},
{
"identity" : "gtm-session-fetcher",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/gtm-session-fetcher.git",
"state" : {
"revision" : "c0ac7575d70050c2973ba2318bd5af47f8e8153a",
"version" : "5.3.0"
}
},
{
"identity" : "interop-ios-for-google-sdks",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/interop-ios-for-google-sdks.git",
"state" : {
"revision" : "040d087ac2267d2ddd4cca36c757d1c6a05fdbfe",
"version" : "101.0.0"
}
},
{
"identity" : "leveldb",
"kind" : "remoteSourceControl",
"location" : "https://github.com/firebase/leveldb.git",
"state" : {
"revision" : "a0bc79961d7be727d258d33d5a6b2f1023270ba1",
"version" : "1.22.5"
}
},
{
"identity" : "nanopb",
"kind" : "remoteSourceControl",
"location" : "https://github.com/firebase/nanopb.git",
"state" : {
"revision" : "b7e1104502eca3a213b46303391ca4d3bc8ddec1",
"version" : "2.30910.0"
}
},
{
"identity" : "promises",
"kind" : "remoteSourceControl",
"location" : "https://github.com/google/promises.git",
"state" : {
"revision" : "540318ecedd63d883069ae7f1ed811a2df00b6ac",
"version" : "2.4.0"
}
}
],
"version" : 2
}

```