feat(checks): add a safe run_sync() convenience method

[kevinmessiaen]

Problem

The public checks API is async-only. In a normal Python script, users must write asyncio.run(scenario.run()) (and the same applies to Suite and TestCase). This is boilerplate for a common synchronous entry point.

Proposed change

Add run_sync(...) to the public, user-facing runnable types in giskard-checks:

• Scenario
• Suite
• TestCase

Keep the existing async run(...) API unchanged. Use one shared internal helper or mixin so the loop-detection behavior and error message stay consistent.

Example:

result = scenario.run_sync()
results = suite.run_sync(parallel=True, verbose=False)

Required behavior

• In a plain synchronous script or synchronous test, run_sync(*args, **kwargs) returns the same result as asyncio.run(self.run(*args, **kwargs)).
• It forwards all positional and keyword arguments exactly to run.
• It must work from a non-main thread that has no running event loop.
• If called while an asyncio event loop is already running (Jupyter/IPython notebooks, async pytest tests, async web handlers, or an async def function), it must raise a clear RuntimeError that tells the user to call await obj.run(...) instead. It must not patch, nest, stop, or run the existing loop in another thread.
• Do not add nest_asyncio or another runtime dependency.

Docs and tests

• Update the checks README next to the current async script example. Show both forms:

  # Script / no running event loop
  result = scenario.run_sync()
  
  # Notebook, pytest async test, or async function
  result = await scenario.run()

• Add unit tests for each supported type: successful synchronous execution, argument forwarding, result/exception propagation, and the active-loop error path.

Out of scope

• Do not change run() to be synchronous.
• Do not try to make synchronous blocking calls work inside a running loop.
• Do not expand this issue to low-level Check.run() or agent workflow/tool APIs; those may need a separate API-design decision.

[linear-code]

OSS-140