Remove OPENAI_BASE_URL config fallback (#16720)

The `OPENAI_BASE_URL` environment variable has been a significant
support issue, so we decided to deprecate it in favor of an
`openai_base_url` config key. We've had the deprecation warning in place
for about a month, so users have had time to migrate to the new
mechanism. This PR removes support for `OPENAI_BASE_URL` entirely.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__feedback_upload_consent_popup.snap

@@ -9,8 +9,8 @@ expression: popup
     • codex-connectivity-diagnostics.txt
 
   Connectivity diagnostics
-    - OPENAI_BASE_URL is set and may affect connectivity.
-      - OPENAI_BASE_URL = hello
+    - Proxy environment variables are set and may affect connectivity.
+      - HTTPS_PROXY = hello
 
 › 1. Yes  Share the current Codex session logs with the team for
           troubleshooting.

codex-rs/tui/src/chatwidget/tests/popups_and_settings.rs

@@ -1763,8 +1763,9 @@ async fn feedback_upload_consent_popup_snapshot() {
         chat.current_rollout_path.clone(),
         &codex_feedback::feedback_diagnostics::FeedbackDiagnostics::new(vec![
             codex_feedback::feedback_diagnostics::FeedbackDiagnostic {
-                headline: "OPENAI_BASE_URL is set and may affect connectivity.".to_string(),
-                details: vec!["OPENAI_BASE_URL = hello".to_string()],
+                headline: "Proxy environment variables are set and may affect connectivity."
+                    .to_string(),
+                details: vec!["HTTPS_PROXY = hello".to_string()],
             },
         ]),
     ));
@@ -1783,8 +1784,9 @@ async fn feedback_good_result_consent_popup_includes_connectivity_diagnostics_fi
         chat.current_rollout_path.clone(),
         &codex_feedback::feedback_diagnostics::FeedbackDiagnostics::new(vec![
             codex_feedback::feedback_diagnostics::FeedbackDiagnostic {
-                headline: "OPENAI_BASE_URL is set and may affect connectivity.".to_string(),
-                details: vec!["OPENAI_BASE_URL = hello".to_string()],
+                headline: "Proxy environment variables are set and may affect connectivity."
+                    .to_string(),
+                details: vec!["HTTPS_PROXY = hello".to_string()],
             },
         ]),
     ));