Support per-tool MCP output limits (#41421)

## What changed

- Add a positive `output_token_limit` setting to each entry under an MCP server's `tools` configuration.
- Apply the most restrictive limit when plugin and user policies overlap, while keeping approval policy independent.
- Carry the effective MCP output budget in conversation history so tool output, post-tool hook responses, and resumed sessions use the same truncation limit.

## Testing

- Cover configuration parsing, serialization, schema validation, and plugin policy merging.
- Cover MCP output below and above the configured limit, post-tool hook responses, and session resume.

GitOrigin-RevId: d0beb4fca9ba6055d9e1d31c137373b465d50d61

codex-rs/core/config.schema.json

@@ -1966,7 +1966,7 @@
     },
     "McpServerToolConfig": {
       "additionalProperties": false,
-      "description": "Per-tool approval settings for a single MCP server tool.",
+      "description": "Per-tool settings for a single MCP server tool.",
       "properties": {
         "approval_mode": {
           "allOf": [
@@ -1975,6 +1975,12 @@
             }
           ],
           "description": "Approval mode for this tool."
+        },
+        "output_token_limit": {
+          "description": "Token budget for this tool's output, before the standard 20% serialization allowance.",
+          "format": "uint",
+          "minimum": 1.0,
+          "type": "integer"
         }
       },
       "type": "object"
@@ -2990,7 +2996,7 @@
           "additionalProperties": {
             "$ref": "#/definitions/McpServerToolConfig"
           },
-          "description": "Per-tool approval settings keyed by tool name.",
+          "description": "Per-tool policy settings keyed by tool name.",
           "type": "object"
         }
       },