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/core/config.schema.json

@@ -2260,6 +2260,10 @@
       },
       "type": "array"
     },
+    "openai_base_url": {
+      "description": "Base URL override for the built-in `openai` model provider.",
+      "type": "string"
+    },
     "oss_provider": {
       "description": "Preferred OSS provider for local models, e.g. \"lmstudio\" or \"ollama\".",
       "type": "string"