Update to Firebase 12.15.0

[github-actions]

The Firebase Apple SDK (12.15.0) is now available. For more details, see
the Firebase Apple SDK release notes.

To install this SDK, see Add Firebase to your project.

What's Changed

• [Core] Require swift-tools-version:6.1 in Package.swift by @andrewheard in [Core] Require swift-tools-version:6.1 in Package.swift firebase/firebase-ios-sdk#16223
• [AI] Rename GeminiModel to GeminiLanguageModel by @andrewheard in [AI] Rename GeminiModel to GeminiLanguageModel firebase/firebase-ios-sdk#16227
• fix(appcheck): remove redundant fac debug token logging by @ncooke3 in fix(appcheck): remove redundant fac debug token logging firebase/firebase-ios-sdk#16197
• feat(appcheck): Add new internal, default provider by @ncooke3 in feat(appcheck): Add new internal, default provider firebase/firebase-ios-sdk#16190
• docs(appcheck): prefer 'AppCheckDebugToken' for debug env var injection by @ncooke3 in docs(appcheck): prefer 'AppCheckDebugToken' for debug env var injection firebase/firebase-ios-sdk#16230
• feat(appcheck): add helpful log for registering debug tokens by @ncooke3 in feat(appcheck): add helpful log for registering debug tokens firebase/firebase-ios-sdk#16232
• [AI] Add Hashable conformance to model config types by @andrewheard in [AI] Add Hashable conformance to model config types firebase/firebase-ios-sdk#16233
• [AI] Add public init for GeminiLanguageModel by @andrewheard in [AI] Add public init for GeminiLanguageModel firebase/firebase-ios-sdk#16234
• feat(ai): add app check dependency by @ncooke3 in feat(ai): add app check dependency firebase/firebase-ios-sdk#16185
• Readme improvements by @paulb777 in Readme improvements firebase/firebase-ios-sdk#16238
• [AI] Revert rename of GeminiModel to GeminiLanguageModel by @andrewheard in [AI] Revert rename of GeminiModel to GeminiLanguageModel firebase/firebase-ios-sdk#16237
• Migrate repo dev content from README to CONTRIBUTING by @paulb777 in Migrate repo dev content from README to CONTRIBUTING firebase/firebase-ios-sdk#16239
• [AI] Add readme banner for FMF adapter by @paulb777 in [AI] Add readme banner for FMF adapter firebase/firebase-ios-sdk#16248
• Analytics 12.15.0 by @htcgh in Analytics 12.15.0 firebase/firebase-ios-sdk#16250
• fix(firestore): remove designated initializers for force_index by @dlarocque in fix(firestore): remove designated initializers for force_index firebase/firebase-ios-sdk#16229
• fix(ai): Handle Xcode 27 LanguageModelSession.Error collision by @andrewheard in fix(ai): Handle Xcode 27 LanguageModelSession.Error collision firebase/firebase-ios-sdk#16257
• feat(appcheck): implement RecaptchaEnterpriseProvider and tests by @ncooke3 in feat(appcheck): implement RecaptchaEnterpriseProvider and tests firebase/firebase-ios-sdk#16155
• [AI] Support SpeechConfig in Text-To-Speech by @milaGGL in [AI] Support SpeechConfig in Text-To-Speech firebase/firebase-ios-sdk#16226
• try building docc docs on swift package index by @morganchen12 in try building docc docs on swift package index firebase/firebase-ios-sdk#16262

Full Changelog: firebase/firebase-ios-sdk@12.14.0...12.15.0