Add openai_base_url config override for built-in provider (#12031)

We regularly get bug reports from users who mistakenly have the
`OPENAI_BASE_URL` environment variable set. This PR deprecates this
environment variable in favor of a top-level config key
`openai_base_url` that is used for the same purpose. By making it a
config key, it will be more visible to users. It will also participate
in all of the infrastructure we've added for layered and managed
configs.

Summary
- introduce the `openai_base_url` top-level config key, update
schema/tests, and route the built-in openai provider through it while
- fall back to deprecated `OPENAI_BASE_URL` env var but warn user of
deprecation when no `openai_base_url` config key is present
- update CLI, SDK, and TUI code to prefer the new config path (with a
deprecated env-var fallback) and document the SDK behavior change

codex-rs/app-server/tests/common/config.rs

@@ -49,6 +49,11 @@ stream_max_retries = 0
 "#
         )
     };
+    let openai_base_url_line = if model_provider_id == "openai" {
+        format!("openai_base_url = \"{server_uri}/v1\"\n")
+    } else {
+        String::new()
+    };
     // Phase 3: write the final config file.
     let config_toml = codex_home.join("config.toml");
     std::fs::write(
@@ -62,6 +67,7 @@ compact_prompt = "{compact_prompt}"
 model_auto_compact_token_limit = {auto_compact_limit}
 
 model_provider = "{model_provider_id}"
+{openai_base_url_line}
 
 [features]
 {feature_entries}