Add missing f-str prefix to log line

- use more compact optional value style with `|`-union
- fix `.flows` typing-only import since we need `MktPair` to be
  immediately defined for use on a `msgspec.Struct` field.
- more "tree-like" warning msg in `.validate()` reporting.

piker/data/_sampling.py

@@ -740,7 +740,7 @@ async def sample_and_broadcast(
 
                                 log.warning(
                                     f'Feed OVERRUN {sub_key}'
-                                    '@{bus.brokername} -> \n'
+                                    f'@{bus.brokername} -> \n'
                                     f'feed @ {chan.uid}\n'
                                     f'throttle = {throttle} Hz'
                                 )

piker/data/feed.py

@@ -786,7 +786,6 @@ async def install_brokerd_search(
 
 @acm
 async def maybe_open_feed(
-
     fqmes: list[str],
     loglevel: str | None = None,
 
@@ -840,7 +839,6 @@ async def maybe_open_feed(
 
 @acm
 async def open_feed(
-
     fqmes: list[str],
 
     loglevel: str|None = None,

piker/data/flows.py

@@ -36,10 +36,10 @@ from ._sharedmem import (
     ShmArray,
     _Token,
 )
+from piker.accounting import MktPair
 
 if TYPE_CHECKING:
-    from ..accounting import MktPair
+    from piker.data.feed import Feed
-    from .feed import Feed
 
 
 class Flume(Struct):

piker/data/validate.py

@@ -113,9 +113,9 @@ def validate_backend(
             )
             if ep is None:
                 log.warning(
-                    f'Provider backend {mod.name} is missing '
+                    f'Provider backend {mod.name!r} is missing '
-                    f'{daemon_name} support :(\n'
+                    f'{daemon_name!r} support?\n'
-                    f'The following endpoint is missing: {name}'
+                    f'|_module endpoint-func missing: {name!r}\n'
                 )
 
     inits: list[