Add end-to-end integration test for Python SDK

[sunilp]

Summary

The Python SDK has 29 unit tests for types and routing logic, but no end-to-end test that spins up an actual delegate and client talking over HTTP.

What to do

1. Create sdk/python/tests/test_integration.py
2. Implement a test delegate using LdpDelegate
3. Start it on a random port (use uvicorn programmatically or a test fixture)
4. Use LdpClient to discover, establish session, and submit a task
5. Verify the response includes correct output and provenance

Why this matters

Without integration tests, we can't catch issues in the HTTP layer, session handshake, or serialization between client and delegate. This is the most important test gap in the SDK.

Hints

• Look at tests/ldp_integration.rs in the Rust crate for the pattern (it does exactly this)
• Use pytest-asyncio for async test support
• Start uvicorn in a background thread or use httpx.ASGITransport to test without a real server

Acceptance criteria

• At least 3 integration tests: discover, submit_task, trust domain rejection
• Tests pass with pytest sdk/python/tests/test_integration.py
• No external dependencies beyond what's in pyproject.toml[dev]