Implement Python SDK runtime and integration suite

[longquanzheng]

Summary

• implement the synchronous Python Client, Worker, Registry, Context, codecs, and gRPC runtime
• expose the shared Rust blob cache through a focused PyO3 binding
• port all 58 Java integration scenarios, workflows, and assertions into sdk-python/tests/integ
• run Python SDK integration CI against dexcli dev
• migrate Python protobuf generation and project workflows to uv

Rationale

The Python SDK now follows the host-native architecture: workflow execution and transport remain Python-native, while Rust is limited to the shared blob-cache implementation. The integration suite mirrors the established Java behavior to keep SDK semantics aligned.

User impact

Python applications can use strongly typed, synchronous Flow, Step, and RPC contracts with a working Client and Worker against Dex. Async handlers are rejected during registry validation.

Validation

• UV_CACHE_DIR=/tmp/dex-python-sdk-uv-cache ./run-integration-tests.sh — 58 passed
• uv run --frozen pytest -q tests/test_contracts.py — 15 passed
• uv run --frozen mypy --strict tests/typecheck_contracts.py tests/integ
• uv run --frozen pyright tests/typecheck_contracts.py tests/integ
• cargo test -p dex-blob-cache-python
• cargo clippy -p dex-blob-cache-python --all-targets -- -D warnings
• cargo fmt --all --check