fix(context-engine): honor assembled prompt authority in precheck (#74255)

Merged via squash.

Prepared head SHA: 650b02380b
Co-authored-by: 100yenadmin <239388517+100yenadmin@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

docs/.generated/plugin-sdk-api-baseline.sha256

@@ -1,2 +1,2 @@
-c1446005a26262d6b817d72493471d11c618b98441fad2014f1cf422bfe64bc9  plugin-sdk-api-baseline.json
-1b7d71eaabcae7d957396e7ff242598ef22b51851bc3fe1f4b58f2c2e5bf1459  plugin-sdk-api-baseline.jsonl
+37787172adf7a55a32097599b4bf5729fc7138c8743c6f4c9d58fc8d01df72a1  plugin-sdk-api-baseline.json
+0ec4957528477832085c638a5f7f691c878ba199f3e81f330f162c27cfd9ebf4  plugin-sdk-api-baseline.jsonl

docs/concepts/context-engine.md

@@ -197,6 +197,17 @@ Required members:
 <ParamField path="systemPromptAddition" type="string">
   Prepended to the system prompt.
 </ParamField>
+<ParamField path="promptAuthority" type='"assembled" | "preassembly_may_overflow"'>
+  Controls which token estimate the runner uses for preemptive overflow
+  prechecks. Defaults to `"assembled"`, which means only the assembled
+  prompt's estimate is checked — appropriate for engines that return a
+  windowed, self-contained context. Set to `"preassembly_may_overflow"` only
+  when your assembled view can hide overflow risk in the underlying
+  transcript; the runner then takes the maximum of the assembled estimate
+  and the pre-assembly (unwindowed) session-history estimate when deciding
+  whether to preemptively compact. Either way, the messages you return are
+  still what the model sees — `promptAuthority` only affects the precheck.
+</ParamField>
 
 `compact` returns a `CompactResult`. When compaction rotates the active
 transcript, `result.sessionId` and `result.sessionFile` identify the successor