[codex] exec-server: stream files in chunks (#28354)

## Why

`fs/readFile` buffers the entire file in one response, which makes large
remote reads expensive and prevents callers from applying backpressure.
We need an opt-in streaming path with bounded block sizes while
preserving the existing single-call API for small and sandboxed reads.

## What changed

- Add `ExecServerClient::stream`, returning a named `FileReadStream`
that implements `futures::Stream` and yields immutable 1 MiB byte
blocks.
- Add internal `fs/open`, `fs/readBlock`, and `fs/close` RPCs.
`fs/readBlock` accepts an explicit offset and length.
- Keep unsandboxed files open between block reads, cap open handles per
connection, and clean them up on EOF, error, stream drop, explicit
close, or connection shutdown.
- Reject platform-sandboxed streaming opens instead of turning the
one-shot sandbox helper into a persistent server. Existing `fs/readFile`
behavior is unchanged.

## Testing

- `just test -p codex-exec-server`
- Integration coverage for 1 MiB chunking, exact block-boundary EOF,
sandbox rejection, and continued reads from the opened file after path
replacement.
- Handle-manager coverage for non-sequential offsets, variable block
lengths, the 128-handle limit, and capacity release after close.

codex-rs/core-plugins/src/provider_tests.rs

@@ -8,6 +8,7 @@ use codex_exec_server::EnvironmentManager;
 use codex_exec_server::ExecutorFileSystem;
 use codex_exec_server::ExecutorFileSystemFuture;
 use codex_exec_server::FileMetadata;
+use codex_exec_server::FileSystemReadStream;
 use codex_exec_server::FileSystemResult;
 use codex_exec_server::FileSystemSandboxContext;
 use codex_exec_server::LOCAL_ENVIRONMENT_ID;
@@ -89,6 +90,14 @@ impl ExecutorFileSystem for SyntheticPluginFileSystem {
         })
     }
 
+    fn read_file_stream<'a>(
+        &'a self,
+        _path: &'a PathUri,
+        _sandbox: Option<&'a FileSystemSandboxContext>,
+    ) -> ExecutorFileSystemFuture<'a, FileSystemReadStream> {
+        Box::pin(async { Self::unsupported() })
+    }
+
     fn write_file<'a>(
         &'a self,
         _path: &'a PathUri,