feat: add Python type stubs (.pyi) for the public API

[zanni098]

Problem

DuckTap has no Python type stubs (.pyi files) for its public API. Users who import ducktap as a library get no IDE autocompletion or type hints in their editor.

Proposed Solution

Add type stubs for the public API surface:

1. src/ducktap-stubs/init.pyi - public API types
2. src/ducktap-stubs/core/spec.pyi - APISpec, Param, Operation, Response types
3. Generate stubs with pyright or mypy --custom-typeshed

Acceptance Criteria

• Type stubs exist for the public API surface
• IDE autocompletion works when importing ducktap
• Stubs are included in the package distribution
• pyright --verifytypes ducktap passes without errors