Resolve session-store method checks against the instance

[shashvat-singham]

Problem

_store_implements resolves the method on type(store), so it only recognises class-level definitions. But SessionStore is a structural Protocol — an implementation assigned on the instance satisfies it just as well, and calling it works fine at runtime. Those stores are nonetheless rejected during pre-flight validation:

class DelegatingStore(SessionStore):
    def __init__(self, inner: SessionStore) -> None:
        self.list_sessions = inner.list_sessions
    ...

validate_session_store_options(
    ClaudeAgentOptions(session_store=DelegatingStore(inner), continue_conversation=True)
)
# ValueError: continue_conversation with session_store requires the store to
# implement list_sessions()

Verified against main, three shapes all wrongly reported as not implementing it:

store	_store_implements	should be
class-level async def list_sessions	True	True
assigned in __init__ (delegation)	False	True
patched with AsyncMock	False	True
genuinely missing	False	False

The AsyncMock row is probably the most likely way to meet this in practice — someone stubbing a store in their own tests gets a ValueError that only appears when continue_conversation=True, pointing at a method their double clearly has.

Change

Look the attribute up on the instance and compare the underlying function against the Protocol default:

impl = getattr(store, method, None)
if impl is None:
    return False
default = getattr(SessionStore, method, None)
return getattr(impl, "__func__", impl) is not default

A bound method is still matched against the Protocol default via __func__, so an unimplemented store is still detected; anything that isn't a bound method (a plain callable assigned on the instance) is compared directly and is never the default. The getattr(impl, ...) also makes the existing impl is None guard meaningful — previously impl was computed and then not used for the decision.

Confirmed the negative case still works: a store without list_sessions is still rejected, and test_continue_conversation_requires_list_sessions still passes.

Tests

Added test_continue_conversation_ok_when_list_sessions_set_on_instance, which fails on main and passes with the change.

$ pytest tests/test_session_store_conformance.py -q
8 failed, 22 passed

The 8 failures are all the [trio] parametrisations and reproduce identically on an unmodified tree here (no trio backend in my env) — unrelated to this change; the [asyncio] side is green.