[codex] Outline ToolExecutor handler bodies

[anp-oai]

Why

We're now discouraging use of async_trait.

Removing use of async_trait from ToolExecutor yields a codex_core debug test build speedup of ~78% (from 227.5s to 50.3s) on my machine.

For ease of reviewing, this is a prefactor to extract trait method implementations to inherent methods. This will prevent changing indentation from creating a huge diff.

What

Outlined existing ToolExecutor::handle bodies into inherent async handle_call methods across core and extension tool handlers.

The trait methods still use async_trait and now delegate to self.handle_call(...).await; handler behavior is unchanged.