perf: preserve conversation prefixes across agent turns

tests/unit/test_plugin_result_delivery.py

@@ -65,7 +65,9 @@ def test_burst_results_reach_responses_once_with_images_before_compression(tmp_p
     actions[2].pop("post_image_name")
     actions[2]["attempts"] = ["Skipped"]
     report = {"status": "failed", "execution": actions}
-    ledger = TranscriptLedger(image_scrub_depth=1, pending_grace_steps=0)
+    ledger = TranscriptLedger(
+        image_scrub_depth=1, pending_grace_steps=0, context_budget_tokens=300_000
+    )
     _stage(ledger, actions)
     ledger.commit_staged(
         step_key="step1",
@@ -84,13 +86,17 @@ def test_burst_results_reach_responses_once_with_images_before_compression(tmp_p
     assert len([item for item in payload if item.get("type") == "function_call_output"]) == 4
     assert str(payload).count(base64.b64encode(image.read_bytes()).decode()) == 3
     assert f"data:{Image.MIME[image_format]};base64," in str(payload)
-    # No duplicate outputs at the following turn; old images are eligible for scrubbing again.
+    # No duplicate outputs or rewritten images on the following ordinary turn.
     ledger.commit_staged(
         validator_result=report, tool_results=plugin_result_messages(report, engine)
     )
     later = ledger.render([])
     assert len([m for m in later if isinstance(m, ToolMessage)]) == 4
+    assert str(later).count(base64.b64encode(image.read_bytes()).decode()) == 3
+    ledger.compact_active()
+    later = ledger.render([])
     assert str(later).count(base64.b64encode(image.read_bytes()).decode()) < 3
+    assert str(messages).count(base64.b64encode(image.read_bytes()).decode()) == 3
 
 
 def test_missing_plugin_screenshot_keeps_real_result_and_reports_missing_evidence(tmp_path):