[mcp] Fix plugin MCP approval policy. (#19537)

Plugin MCP servers are loaded from plugin manifests rather than
top-level `[mcp_servers]`, so their tool approval preferences need to be
stored and applied through the owning plugin config. Without this,
choosing "Always allow" for a plugin MCP tool could write a preference
that was not reliably used on later tool calls.

## Summary
- Add plugin-scoped MCP policy config under
`plugins.<plugin>.mcp_servers`, including server enablement, tool
allow/deny lists, server defaults, and per-tool approval modes.
- Overlay plugin MCP policy onto manifest-provided server configs when
plugins are loaded.
- Route persistent "Always allow" writes for plugin MCP tools back to
the owning `plugins.<plugin>.mcp_servers.<server>.tools.<tool>` config
entry.
- Reload user config after persisting an approval and make the plugin
load cache config-aware so stale plugin MCP policy is not reused after
`config.toml` changes.
- Regenerate the config schema and add coverage for plugin MCP policy
loading, approval lookup, persistence, and stale-cache prevention.

## Testing
- `cargo test -p codex-config`
- `cargo test -p codex-core-plugins`
- `cargo test -p codex-core --lib plugin_mcp`

codex-rs/app-server/src/config_api.rs

@@ -542,6 +542,7 @@ mod tests {
                 },
             }),
             mcp_servers: None,
+            plugins: None,
             apps: None,
             rules: None,
             enforce_residency: Some(CoreResidencyRequirement::Us),
@@ -670,6 +671,7 @@ mod tests {
             feature_requirements: None,
             hooks: None,
             mcp_servers: None,
+            plugins: None,
             apps: None,
             rules: None,
             enforce_residency: None,
@@ -730,6 +732,7 @@ mod tests {
             feature_requirements: None,
             hooks: None,
             mcp_servers: None,
+            plugins: None,
             apps: None,
             rules: None,
             enforce_residency: None,