Add handle_response to normalize @on_method return values

[Garulf]

Summary

• Adds handle_response() function that transparently normalizes what @on_method-decorated functions return — no more boilerplate send_results() calls
• Plugin methods can now yield Result(...), return a List[Result], or use generators; existing methods returning JsonRPCResponse or JsonRPCRequest continue to work unchanged
• Introduces pyflowlauncher/types.py to hold the Method alias, breaking a latent circular import between result.py and plugin.py
• Async generators are collected in EventHandler._await_maybe

New usage

@plugin.on_method
def query(q: str):
    yield Result(title="foo")
    yield Result(title="bar")

@plugin.on_method
async def query(q: str):
    yield Result(title="async result")

@plugin.on_method
def query(q: str):
    return [Result(title="a"), Result(title="b")]

Test plan

• pytest tests/ — all 57 tests pass (15 new in tests/test_response.py)
• Unit tests cover: single Result, List[Result], sync generator (single/multiple/list yields), JsonRPCRequest pass-through, JsonRPCResponse pass-through, None
• Integration tests cover: @plugin.on_method with generator, single return, list return
• Async generator tests: single yield, multiple yields, list yield