Fix CI: examples import + missing async provenance tool

[Scottcjn]

Summary

CI on main has been failing for two independent reasons:

1. tests/test_examples.py imports the top-level examples package, but examples/ is deliberately not part of the installed distribution (it's a runnable demo, not library code). In a clean pip install -e ".[test]" environment like CI's, the repo root is never on sys.path, so the import fails with ModuleNotFoundError: No module named 'examples'. This happened to pass on my own machine only because my shell sets an empty PYTHONPATH entry that puts cwd on the path — not something CI has. Fixed by adding pythonpath = ["."] under [tool.pytest.ini_options] in pyproject.toml, so pytest puts the repo root on sys.path itself, independent of the environment.
2. test_async_tools_match_sync_tool_names failed because get_rustchain_tools (sync) got a rustchain_provenance tool when RIP-0310 support was added (commit 2406aa6), but get_async_rustchain_tools (async) never did — the async client didn't even have a provenance() method. Fixed by adding beacon_agents(), beacon_contracts() and provenance() to AsyncRustChainClient (async twins of the existing sync client methods) plus a matching _AsyncProvenanceTool, so the async tool surface actually mirrors the sync one, as the docstring already claimed.

Test plan

• Reproduced both failures locally in a clean venv with PYTHONPATH unset, matching CI's pip install -e ".[test]" + pytest -q exactly.
• After the fix, pytest -q passes with 33/33 on Python 3.11 and 3.12 in clean venvs.
• No tests were skipped or deleted; the fix addresses the actual root causes (a packaging/sys.path gap and a genuinely missing async method).