Define exec-server protocol types for streamed file writes (#49778)

## What changed

- Add `read` and `replace` modes to `fs/open`, defaulting omitted modes to `read` for existing callers.
- Define `fs/writeBlock` request and response types for offset-based writes, with a maximum decoded chunk size of 1 MiB.
- Add the `fileWriteStreaming` capability, defaulting to `false`. Keep writable streams unsupported in the server and explicitly reject replacement opens.

## Testing

Add a protocol test verifying that legacy `fs/open` requests without a mode deserialize as read-only opens.

GitOrigin-RevId: 4d3997717e821cd1f62df12a03c8eec36332a66d

codex-rs/exec-server/src/server/file_system_handler.rs

@@ -28,6 +28,7 @@ use crate::protocol::FsCreateDirectoryParams;
 use crate::protocol::FsCreateDirectoryResponse;
 use crate::protocol::FsGetMetadataParams;
 use crate::protocol::FsGetMetadataResponse;
+use crate::protocol::FsOpenMode;
 use crate::protocol::FsOpenParams;
 use crate::protocol::FsOpenResponse;
 use crate::protocol::FsReadBlockParams;
@@ -122,6 +123,12 @@ impl FileSystemHandler {
         params: FsOpenParams,
     ) -> Result<FsOpenResponse, JSONRPCErrorError> {
         validate_file_handle_id(&params.handle_id)?;
+        // TODO(anp): Enable replacement opens when writable file streams are implemented.
+        if params.mode == FsOpenMode::Replace {
+            return Err(invalid_request(
+                "exec-server does not support writable file streams".to_string(),
+            ));
+        }
         let file = self
             .file_system
             .open_file_for_read(&params.path, params.sandbox.as_ref())