feat: extend codegen to generate client API handler types

Add support for the 'client' section of api.schema.json:
- Generate typed handler interfaces (e.g., SessionStoreHandler)
- Generate ClientApiHandlers composite interface
- Generate registerClientApiHandlers() registration function
- Handle methods with no result type (void return)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS