Upgrade Genkit packages to the current flutter_gemma API

The Genkit integration packages (packages/genkit_flutter_gemma, packages/genkit_hybrid) wrap flutter_gemma's public API. When that API changes, they must be realigned or they break. In the monorepo they consume flutter_gemma as an in-workspace path dependency, so a core change can break them in the same commit — run this whenever a PR changes the public InferenceModel / InferenceChat / EmbeddingModel / Message / ModelResponse / enum surface.

Follow these 5 phases sequentially. Ask the user before deciding which new features to support (Phase 3).

Paths below are relative to the repo root. Core package = packages/flutter_gemma; genkit package = packages/genkit_flutter_gemma (mirror for genkit_hybrid). REPO_ROOT = monorepo root, PKG_DIR=packages/genkit_flutter_gemma, EXAMPLE_DIR=$PKG_DIR/example.

Phase 1: Reconnaissance

1. Identify what changed in the core public API. In the monorepo flutter_gemma is a path dep (not pub), so don't rely on dart pub outdated — instead read the diff / CHANGELOG of packages/flutter_gemma:
   • git log --oneline -- packages/flutter_gemma/lib/flutter_gemma_interface.dart packages/flutter_gemma/lib/core
   • packages/flutter_gemma/CHANGELOG.md (top entry)
2. Summarize for the user:
   • Breaking changes (removed/renamed APIs, changed signatures)
   • New APIs (new parameters, methods, enums, classes)
   • Bug fixes / Deprecations
3. melos bootstrap (or dart pub get in PKG_DIR + EXAMPLE_DIR).

Phase 2: Fix Compilation

1. cd packages/genkit_flutter_gemma && dart analyze (repeat for genkit_hybrid).
2. Fix all compilation errors. Files that depend on flutter_gemma's API:
   • lib/src/flutter_gemma_runtime.dart — FlutterGemma.getActiveModel(), getActiveEmbedder() signatures
   • lib/src/flutter_gemma_model.dart — InferenceModel.createChat() signature, ModelResponse pattern matching, InferenceChat methods
   • lib/src/flutter_gemma_embedder.dart — EmbeddingModel.generateEmbeddings(), PreferredBackend enum
   • lib/src/converters/request_converter.dart — Message constructors (.withImage, .withAudio, .toolCall, .toolResponse)
   • lib/src/converters/response_converter.dart — ModelResponse subtypes (TextResponse, FunctionCallResponse, ParallelFunctionCallResponse, ThinkingResponse)
   • lib/src/converters/tool_converter.dart — Tool constructor
   • lib/src/flutter_gemma_plugin.dart — ModelType, ModelFileType enums
   • test/src/fake_runtime.dart — FakeInferenceModel, FakeInferenceChat, FakeEmbeddingModel must match upstream abstract-class signatures (see genkit CLAUDE.md)
3. Repeat dart analyze until clean (0 issues).

Phase 3: Support New Features

Based on Phase 1, decide with the user which new APIs to support. Typical integration points:

New parameter in createChat() / createSession()

1. Add field to $FlutterGemmaModelOptions in lib/src/flutter_gemma_options.dart
2. Manually update lib/src/flutter_gemma_options.g.dart (constructor param, field with @override, fromJson, toJson, jsonSchema() entry)
3. Extract from config + pass in lib/src/flutter_gemma_model.dart (_executeGeneration)
4. Update FakeInferenceModel.createChat() in test/src/fake_runtime.dart (add param, store in last* field for assertions)

New enum value (e.g. ModelFileType)

• Update comments in lib/src/flutter_gemma_plugin.dart (FlutterGemmaModelConfig.fileType)

New method on InferenceChat / InferenceModel

• Add override in FakeInferenceChat / FakeInferenceModel in test/src/fake_runtime.dart

Changed model capabilities

• Update supports map in lib/src/flutter_gemma_plugin.dart (list() method)

Changed message handling

• Update converters in lib/src/converters/; update extractSystemInstruction() if system-message handling changed

IMPORTANT: *.g.dart here is manually maintained (build_runner not used by default). Always hand-update it when changing the schema.

Phase 4: Tests and Review

1. Update existing tests that verify changed behavior.
2. Add tests for each new feature from Phase 3.
3. flutter test (in PKG_DIR) — all must pass.
4. Run /review-pr for comprehensive review; fix what it surfaces.

Phase 5: Finalize

1. Bump version in PKG_DIR/pubspec.yaml (minor for breaking behavior, patch for fixes/non-breaking additions).
2. Update PKG_DIR/CHANGELOG.md (new section at top: breaking **Breaking**: / new features / fixes).
3. Update PKG_DIR/README.md (options table, known limitations, quick-start if usage changed).
4. Verify: dart analyze && flutter test && dart pub publish --dry-run (all clean).
5. The release itself goes through the release skill (genkit packages release in lockstep with the monorepo) — do not publish standalone.