fix(core): ensure compaction is more reliable, add reserve token buffer to ensure that input window has enough room to compact (#12924)

Co-authored-by: James Lal <james@littlebearlabs.io>

packages/opencode/src/session/processor.ts

@@ -342,6 +342,9 @@ export namespace SessionProcessor {
               stack: JSON.stringify(e.stack),
             })
             const error = MessageV2.fromError(e, { providerID: input.model.providerID })
+            if (MessageV2.ContextOverflowError.isInstance(error)) {
+              // TODO: Handle context overflow error
+            }
             const retry = SessionRetry.retryable(error)
             if (retry !== undefined) {
               attempt++