inline hostname resolution for remote sandbox config (#19739)

# Why

Requirements support host-specific
`remote_sandbox_config.hostname_patterns`, but config loading previously
resolved and passed the system hostname through every config-loading
path even when no requirements layer used `remote_sandbox_config`. On
machines where hostname lookup is slow, startup and app-server config
reads paid for a feature that was not active.

We only need the hostname when a requirements layer actually declares
`remote_sandbox_config`, so this moves hostname resolution to the single
requirements merge point and keeps all other config callers unaware of
hostname matching.

# What

- Removed the eager `host_name` plumbing from
`load_config_layers_state`, `load_requirements_toml`, `ConfigBuilder`,
app-server `ConfigManager`, network proxy loading, and related call
sites.
- Resolve the hostname inside
`merge_requirements_with_remote_sandbox_config` only when the incoming
requirements contain `remote_sandbox_config`.

codex-rs/config/src/config_requirements.rs

@@ -842,10 +842,10 @@ pub enum ResidencyRequirement {
 
 impl ConfigRequirementsToml {
     pub fn apply_remote_sandbox_config(&mut self, hostname: Option<&str>) {
-        let Some(hostname) = hostname.and_then(normalize_hostname) else {
+        let Some(remote_sandbox_config) = self.remote_sandbox_config.as_ref() else {
             return;
         };
-        let Some(remote_sandbox_config) = self.remote_sandbox_config.as_ref() else {
+        let Some(hostname) = hostname.and_then(normalize_hostname) else {
             return;
         };
         let Some(matched_config) = remote_sandbox_config