Await the session pool in notify and three neighbours ...

PgSessionStore._get_pool() is async, and four call sites passed its coroutine
straight into a store constructor, so the first query inside died with
"'coroutine' object has no attribute 'fetchrow'": notify always failed, the
reaction runner never got past reading its settings, synapse_instructions was
unusable, and the BYOK resolver caught the AttributeError and silently fell
back to the default credential.

The tests missed it because each one mocked the store or the pool provider
away; the new ones run on a fake asyncpg pool with nothing faked below the tool.

synapse reactions W2: hidden dispatcher profile + synapse_instructions tool