Streamline thread resume and fork handlers (#19495)

## Why

Thread resume and fork had some of the deepest error-handling
indentation in this area because helpers emitted request errors
directly. Returning those failures gives the handlers a single request
boundary while preserving the async pending-resume behavior.

## What Changed

- Converted thread resume helpers in
`codex-rs/app-server/src/codex_message_processor.rs` to return `Result`
values for validation and view loading failures.
- Applied the same pattern to thread fork request handling.
- Simplified pending resume error construction by using the shared
JSON-RPC error helpers.

## Verification

- `cargo check -p codex-app-server`
- `cargo test -p codex-app-server --test all v2::thread_resume --
--test-threads=1`
- `cargo test -p codex-app-server --test all v2::thread_fork --
--test-threads=1`

Author: pakrym-oai