[codex] Split app-server request processors

[pakrym-oai]

Why

The app-server request path had grown around a large CodexMessageProcessor plus separate API wrapper/helper modules. That made the dependency graph hard to see and forced unrelated request families to share broad processor state.

This PR makes the split mechanical and command-prefix oriented so request families own only the dependencies they use.

What changed

• Replaced CodexMessageProcessor with command-prefix request processors under app-server/src/request_processors/.
• Removed the old config, device-key, external-agent-config, and fs API wrapper files by moving their API handling into processors.
• Split apps, plugins, marketplace, catalog, account, MCP, command exec, fs, git, feedback, thread, turn, thread goals, and Windows sandbox handling into dedicated processors.
• Kept shared lifecycle, summary conversion, token usage replay, and shared error mapping only where multiple processors use them; single-use helpers were inlined into their owning processor.
• Removed the fallback processor path and moved processor tests to _tests files.

Validation

• cargo test -p codex-app-server
• cargo check -p codex-app-server
• just fix -p codex-app-server