[codex] Use shared app-server JSON-RPC error helpers

[pakrym-oai]

Why

App-server had repeated hand-built JSON-RPC error objects for standard error shapes. Using the shared helpers keeps the common invalid_request, invalid_params, and internal_error construction in one place and reduces the chance of new call sites drifting from the common error payload shape.

What changed

• Replaced manual standard JSON-RPC error object creation with internal_error(...), invalid_request(...), and invalid_params(...) across app-server request processors and runtime paths.
• Removed local duplicate helper definitions from search and review request handling.
• Preserved existing structured data payloads by creating the shared helper error first and then attaching the existing metadata.
• Left custom non-standard errors and raw error-code assertions intact.

Validation

• cargo test -p codex-app-server