test: add tests for thinking

CRUSH.md

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+# CRUSH.md - Fantasy AI SDK
+
+## Build/Test/Lint Commands
+- **Build**: `go build ./...`
+- **Test all**: `task test` or `go test ./... -count=1`
+- **Test single**: `go test -run TestName ./package -v`
+- **Test with args**: `task test -- -v -run TestName`
+- **Lint**: `task lint` or `golangci-lint run`
+- **Format**: `task fmt` or `gofmt -s -w .`
+- **Modernize**: `task modernize` or `modernize -fix ./...`
+
+## Code Style Guidelines
+- **Package naming**: lowercase, single word (ai, openai, anthropic, google)
+- **Imports**: standard library first, then third-party, then local packages
+- **Error handling**: Use custom error types with structured fields, wrap with context
+- **Types**: Use type aliases for function signatures (`type Option = func(*options)`)
+- **Naming**: CamelCase for exported, camelCase for unexported
+- **Constants**: Use const blocks with descriptive names (ProviderName, DefaultURL)
+- **Structs**: Embed anonymous structs for composition (APICallError embeds *AIError)
+- **Functions**: Return error as last parameter, use context.Context as first param
+- **Testing**: Use testify/assert, table-driven tests, recorder pattern for HTTP mocking
+- **Comments**: Godoc format for exported functions, explain complex logic inline
+- **JSON**: Use struct tags for marshaling, handle empty values gracefully
+
+## Project Structure
+- `/ai` - Core AI abstractions and agent logic
+- `/openai`, `/anthropic`, `/google` - Provider implementations
+- `/providertests` - Cross-provider integration tests with VCR recordings
+- `/examples` - Usage examples for different patterns