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/suite/v2/compaction.rs

@@ -158,15 +158,7 @@ async fn auto_compaction_remote_emits_started_and_completed_items() -> Result<()
         AuthCredentialsStoreMode::File,
     )?;
 
-    let server_base_url = format!("{}/v1", server.uri());
-    let mut mcp = McpProcess::new_with_env(
-        codex_home.path(),
-        &[
-            ("OPENAI_BASE_URL", Some(server_base_url.as_str())),
-            ("OPENAI_API_KEY", None),
-        ],
-    )
-    .await?;
+    let mut mcp = McpProcess::new_with_env(codex_home.path(), &[("OPENAI_API_KEY", None)]).await?;
     timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
 
     let thread_id = start_thread(&mut mcp).await?;