Streamline review and feedback handlers (#19498)

## Why

The remaining review, interrupt, fuzzy search, feedback, and git-diff
handlers still had local send-error branches that obscured otherwise
simple request handling. This final slice flattens those handlers
without changing the public protocol behavior.

## What Changed

- Streamlined review start, turn interrupt, fuzzy search session,
feedback upload, and git diff handlers in
`codex-rs/app-server/src/codex_message_processor.rs`.
- Converted validation and upload failures into returned JSON-RPC errors
where that avoids nested `send_error`/`return` blocks.
- Left unrelated sandbox setup and notification code untouched.

## Verification

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

Author: pakrym-oai