feat: carry the per-call tier down to the transport that must send it

The Transport port took the request apart into five arguments, so a tier
placed on ChatRequest could never reach _build_payload: the field was set,
read by nobody, and silently ignored - the exact shape of failure that sent
downstream to extra_body in the first place.

complete() now takes reasoning_effort with no default, matching the
TelemetryRecorder convention: a default would turn a missing hand-off into
a silent 'no opinion'. All four fakes move with it, since @runtime_checkable
checks method names and not signatures.

EmbeddingTransport and OcrTransport are deliberately left alone - they have
no reasoning semantics - and a test now holds that line.

_build_payload drops its inline sugar conversion for effective_effort(), so
the guard and the hot path share one judgement, and passes the source's
effort_fallback for the same reason.

tests/unit/test_backpressure.py

@@ -210,7 +210,7 @@ class ClockAdvancingTransport:
         self.clock = clock
         self.calls = []
 
-    async def complete(self, *, messages, source, stream, overlay, call_id):
+    async def complete(self, *, messages, source, stream, overlay, call_id, reasoning_effort):
         self.calls.append((source.name, call_id))
         advance, action = self.script.pop(0)
         self.clock.advance(advance)