chore: move test helper functions to conftest.py

[ryanking13]

Moves test helper functions from test_bindings.py to conftest.py so that we can reuse them in other test suites (e.g. in #147)

[ask-bonk]

I've reviewed the entire PR thoroughly. The changes are a clean mechanical refactoring:

• Helper functions, types, and fixtures moved from test_bindings.py to conftest.py
• Functions appropriately generalized (e.g., discover_suites takes src_dir parameter, dev_server takes worker_project_dir fixture)
• Type/function names generalized from binding-specific to generic (e.g., BindingTestResult -> InWorkerTestResult)
• The worker_project_dir fixture override pattern is correct pytest behavior
• @functools.cache on get_suite_results is safe across modules since cache keys include the URL with unique port
• No behavioral changes, no logic bugs, no security concerns

LGTM

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