Add iOS 27 custom language model support

[prasadpamidi]

Summary

• centralize Foundation Models session construction for system and injected models
• accept any iOS 27 LanguageModel, including Core AI-backed models, with capability validation
• add provider tests, an isolated Core AI device proof, and integration documentation
• prepare the 0.12.0 release metadata

Quality checks

• bundle exec fastlane quality — SwiftFormat clean, 0 serious SwiftLint violations
• bundle exec fastlane package_tests — 245 tests passed
• git diff --check

Test plan

• verify the existing system-model provider still checks system availability
• verify injected models build sessions through the shared factory
• verify declared tool, structured-output, and vision capabilities are rejected when unavailable
• compile and run the full Swift package test suite

Release

• updates AriaInfo.version to 0.12.0
• adds 0.12.0 release notes to CHANGELOG.md

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