Make the optional MCP startup grace configurable (#41199)

## What changed

- Add `mcp_optional_startup_grace_ms` with a default of 1,000 ms to control how long tool catalog capture waits for optional MCP servers.
- Treat a value of `0` as disabling the shared grace, so optional servers use their configured `startup_timeout_sec` instead.
- Apply updated grace values during runtime and MCP configuration refreshes, and reset cached startup deadlines when the configured duration changes.

## Testing

- Cover custom, disabled, refreshed, and shared startup grace behavior in MCP connection and core integration tests.

GitOrigin-RevId: 936196215ed6e0a29c9ec72cfee17663978df8ef

codex-rs/core/config.schema.json

@@ -6284,6 +6284,12 @@
       "default": null,
       "description": "Preferred backend for storing MCP OAuth credentials. keyring: Use an OS-specific keyring service. https://github.com/openai/codex/blob/main/codex-rs/rmcp-client/src/oauth.rs#L2 file: Use a file in the Codex home directory. auto (default): Use the OS-specific keyring service if available, otherwise use a file."
     },
+    "mcp_optional_startup_grace_ms": {
+      "description": "Milliseconds to wait for optional MCP servers while building the initial tool catalog.\n\nDefaults to 1000. Set to 0 to disable the shared grace and wait for each server's configured `startup_timeout_sec` instead.",
+      "format": "uint64",
+      "minimum": 0.0,
+      "type": "integer"
+    },
     "mcp_servers": {
       "additionalProperties": {
         "$ref": "#/definitions/RawMcpServerConfig"