refactor: simplify linkIdentityWithIdToken implementation

- Remove private _signInWithIdToken helper method
- Inline the implementation directly in linkIdentityWithIdToken
- Update test to include Authorization header in expected request
- Add session storage setup for linkIdentityWithIdToken test

Author: grdsdev

Sources/Auth/AuthClient.swift

@@ -380,15 +380,7 @@ public actor AuthClient {
   /// The ID token is verified for validity and a new session is established.
   @discardableResult
   public func signInWithIdToken(credentials: OpenIDConnectCredentials) async throws -> Session {
-    try await _signInWithIdToken(credentials: credentials, linkIdentity: false)
-  }
-
-  private func _signInWithIdToken(credentials: OpenIDConnectCredentials, linkIdentity: Bool)
-    async throws -> Session
-  {
-    var credentials = credentials
-    credentials.linkIdentity = linkIdentity
-    return try await _signIn(
+    try await _signIn(
       request: .init(
         url: configuration.url.appendingPathComponent("token"),
         method: .post,
@@ -1193,7 +1185,17 @@ public actor AuthClient {
   public func linkIdentityWithIdToken(
     credentials: OpenIDConnectCredentials
   ) async throws -> Session {
-    try await _signInWithIdToken(credentials: credentials, linkIdentity: true)
+    var credentials = credentials
+    credentials.linkIdentity = true
+    return try await _signIn(
+      request: .init(
+        url: configuration.url.appendingPathComponent("token"),
+        method: .post,
+        query: [URLQueryItem(name: "grant_type", value: "id_token")],
+        headers: [.authorization: "Bearer \(session.accessToken)"],
+        body: configuration.encoder.encode(credentials)
+      )
+    )
   }
 
   /// Links an OAuth identity to an existing user.

Tests/AuthTests/AuthClientTests.swift

@@ -495,6 +495,7 @@ final class AuthClientTests: XCTestCase {
       #"""
       curl \
       	--request POST \
+      	--header "Authorization: Bearer accesstoken" \
       	--header "Content-Length: 166" \
       	--header "Content-Type: application/json" \
       	--header "X-Client-Info: auth-swift/0.0.0" \
@@ -508,6 +509,8 @@ final class AuthClientTests: XCTestCase {
 
     let sut = makeSUT()
 
+    Dependencies[sut.clientID].sessionStorage.store(.validSession)
+
     try await sut.linkIdentityWithIdToken(
       credentials: OpenIDConnectCredentials(
         provider: .apple,