- Add publishable_client_key param to BaseAPIClient, StackServerApp, AsyncStackServerApp
- Include x-stack-publishable-client-key header when key is provided
- Change ExplicitTokenStore defaults from empty string to None
- Remove empty string fallback in resolve_token_store dict branch
- BaseAPIClient generic base with header construction, URL building,
response parsing via x-stack-actual-status and x-stack-known-error
- SyncAPIClient wrapping httpx.Client with context manager support
- AsyncAPIClient wrapping httpx.AsyncClient with async context manager
- Exponential backoff retry for idempotent methods (GET/PUT/DELETE)
- 429 rate limit handling with Retry-After header support
- POST/PUT/PATCH with no body sends {} as JSON
- All 26 tests passing