Add ephemeral turn context support

Introduce app-server ephemeralContext, render it as additional_context_for_this_turn fragments, and strip it correctly across contextual-user diffing and compaction flows.

Regenerate app-server schema fixtures and add coverage for model-visible layout, compaction, oversize validation, and resumed/forked context reconstruction.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -6828,6 +6828,23 @@
         ],
         "type": "object"
       },
+      "EphemeralContext": {
+        "properties": {
+          "text": {
+            "description": "Free-form text payload for additional context sent with one turn.",
+            "type": "string"
+          },
+          "title": {
+            "description": "Human-readable title for additional context sent with one turn.",
+            "type": "string"
+          }
+        },
+        "required": [
+          "text",
+          "title"
+        ],
+        "type": "object"
+      },
       "ErrorNotification": {
         "$schema": "http://json-schema.org/draft-07/schema#",
         "properties": {
@@ -13206,6 +13223,16 @@
             ],
             "description": "Override the reasoning effort for this turn and subsequent turns."
           },
+          "ephemeralContext": {
+            "description": "Additional model-visible context for this turn, such as editor or IDE state. This context is not re-injected automatically after compaction, so clients should send a fresh snapshot on each turn instead of expecting it to carry forward automatically.",
+            "items": {
+              "$ref": "#/definitions/v2/EphemeralContext"
+            },
+            "type": [
+              "array",
+              "null"
+            ]
+          },
           "input": {
             "items": {
               "$ref": "#/definitions/v2/UserInput"