Allow ledger passes to ignore (symcache) unknown fqmes

For example in the paper-eng, if you have a backend that doesn't fully
support a symcache (yet) it's handy to be able to ignore processing
other paper-eng txns when all you care about at the moment is the
simulated symbol.

piker/accounting/_pos.py

@@ -30,7 +30,8 @@ from types import ModuleType
 from typing import (
     Any,
     Iterator,
-    Generator
+    Generator,
+    TYPE_CHECKING,
 )
 
 import pendulum
@@ -59,8 +60,10 @@ from ..clearing._messages import (
     BrokerdPosition,
 )
 from piker.types import Struct
-from piker.data._symcache import SymbologyCache
-from ..log import get_logger
+from piker.log import get_logger
+
+if TYPE_CHECKING:
+    from piker.data._symcache import SymbologyCache
 
 log = get_logger(__name__)
 
@@ -493,6 +496,17 @@ class Account(Struct):
 
         _mktmap_table: dict[str, MktPair] | None = None,
 
+        only_require: list[str]|None = None,
+        # ^list of fqmes that are "required" to be processed from
+        # this ledger pass; we often don't care about others and
+        # definitely shouldn't always error in such cases.
+        # (eg. broker backend loaded that doesn't yet supsport the
+        # symcache but also, inside the paper engine we don't ad-hoc
+        # request `get_mkt_info()` for every symbol in the ledger,
+        # only the one for which we're simulating against).
+        # TODO, not sure if there's a better soln for this, ideally
+        # all backends get symcache support afap i guess..
+
     ) -> dict[str, Position]:
         '''
         Update the internal `.pps[str, Position]` table from input
@@ -535,11 +549,25 @@ class Account(Struct):
                 if _mktmap_table is None:
                     raise
 
+                required: bool = fqme in only_require
                 # XXX: caller is allowed to provide a fallback
                 # mktmap table for the case where a new position is
                 # being added and the preloaded symcache didn't
                 # have this entry prior (eg. with frickin IB..)
-                mkt = _mktmap_table[fqme]
+                if (
+                    not (mkt := _mktmap_table.get(fqme))
+                    and
+                    required
+                ):
+                    raise
+
+                elif not required:
+                    continue
+
+                else:
+                    # should be an entry retreived somewhere
+                    assert mkt
+
 
             if not (pos := pps.get(bs_mktid)):
 
@@ -656,7 +684,7 @@ class Account(Struct):
     def write_config(self) -> None:
         '''
         Write the current account state to the user's account TOML file, normally
-        something like ``pps.toml``.
+        something like `pps.toml`.
 
         '''
         # TODO: show diff output?

piker/clearing/_paper_engine.py

@@ -653,6 +653,7 @@ async def open_trade_dialog(
                 # in) use manually constructed table from calling
                 # the `.get_mkt_info()` provider EP above.
                 _mktmap_table=mkt_by_fqme,
+                only_require=list(mkt_by_fqme),
             )
 
             pp_msgs: list[BrokerdPosition] = []