Update all dependencies and migrate to new Google genai SDK

- Update all Go dependencies to latest versions
- Migrate from github.com/google/generative-ai-go/genai to google.golang.org/genai
- Fix google.go to use the new SDK API (NewPartFromText, NewContentFromParts, etc.)
- Update schema package imports to use the new genai package
- Add CLAUDE.md with README maintenance guideline

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,25 @@
+# CLAUDE.md for go-llm
+
+## Build and Test Commands
+- Build project: `go build ./...`
+- Run all tests: `go test ./...`
+- Run specific test: `go test -v -run <TestName> ./...`
+- Tidy dependencies: `go mod tidy`
+
+## Code Style Guidelines
+- **Indentation**: Use standard Go tabs for indentation.
+- **Naming**: 
+  - Use `camelCase` for internal/private variables and functions.
+  - Use `PascalCase` for exported types, functions, and struct fields.
+  - Interface names should be concise (e.g., `LLM`, `ChatCompletion`).
+- **Error Handling**:
+  - Always check and handle errors immediately.
+  - Wrap errors with context using `fmt.Errorf("%w: ...", err)`.
+  - Use the project's internal `Error` struct in `error.go` when differentiating between error types is needed.
+- **Project Structure**:
+  - `llm.go`: Contains core interfaces (`LLM`, `ChatCompletion`) and shared types (`Message`, `Role`, `Image`).
+  - Provider implementations are in `openai.go`, `anthropic.go`, and `google.go`.
+  - Schema definitions for tool calling are in the `schema/` directory.
+- **Imports**: Organize imports into groups: standard library, then third-party libraries.
+- **Documentation**: Use standard Go doc comments for exported symbols.
+- **README.md**: The README.md file should always be kept up to date with any significant changes to the project.