[MCP] Add support for streamable http servers with codex mcp add and replace bearer token handling (#4904)

1. You can now add streamable http servers via the CLI
2. As part of this, I'm also changing the existing bearer_token plain
text config field with ane env var

```
mcp add github --url https://api.githubcopilot.com/mcp/ --bearer-token-env-var=GITHUB_PAT
```

codex-rs/cli/tests/mcp_add_remove.rs

@@ -93,3 +93,116 @@ async fn add_with_env_preserves_key_order_and_values() -> Result<()> {
 
     Ok(())
 }
+
+#[tokio::test]
+async fn add_streamable_http_without_manual_token() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    let mut add_cmd = codex_command(codex_home.path())?;
+    add_cmd
+        .args(["mcp", "add", "github", "--url", "https://example.com/mcp"])
+        .assert()
+        .success();
+
+    let servers = load_global_mcp_servers(codex_home.path()).await?;
+    let github = servers.get("github").expect("github server should exist");
+    match &github.transport {
+        McpServerTransportConfig::StreamableHttp {
+            url,
+            bearer_token_env_var,
+        } => {
+            assert_eq!(url, "https://example.com/mcp");
+            assert!(bearer_token_env_var.is_none());
+        }
+        other => panic!("unexpected transport: {other:?}"),
+    }
+
+    assert!(!codex_home.path().join(".credentials.json").exists());
+    assert!(!codex_home.path().join(".env").exists());
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn add_streamable_http_with_custom_env_var() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    let mut add_cmd = codex_command(codex_home.path())?;
+    add_cmd
+        .args([
+            "mcp",
+            "add",
+            "issues",
+            "--url",
+            "https://example.com/issues",
+            "--bearer-token-env-var",
+            "GITHUB_TOKEN",
+        ])
+        .assert()
+        .success();
+
+    let servers = load_global_mcp_servers(codex_home.path()).await?;
+    let issues = servers.get("issues").expect("issues server should exist");
+    match &issues.transport {
+        McpServerTransportConfig::StreamableHttp {
+            url,
+            bearer_token_env_var,
+        } => {
+            assert_eq!(url, "https://example.com/issues");
+            assert_eq!(bearer_token_env_var.as_deref(), Some("GITHUB_TOKEN"));
+        }
+        other => panic!("unexpected transport: {other:?}"),
+    }
+    Ok(())
+}
+
+#[tokio::test]
+async fn add_streamable_http_rejects_removed_flag() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    let mut add_cmd = codex_command(codex_home.path())?;
+    add_cmd
+        .args([
+            "mcp",
+            "add",
+            "github",
+            "--url",
+            "https://example.com/mcp",
+            "--with-bearer-token",
+        ])
+        .assert()
+        .failure()
+        .stderr(contains("--with-bearer-token"));
+
+    let servers = load_global_mcp_servers(codex_home.path()).await?;
+    assert!(servers.is_empty());
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn add_cant_add_command_and_url() -> Result<()> {
+    let codex_home = TempDir::new()?;
+
+    let mut add_cmd = codex_command(codex_home.path())?;
+    add_cmd
+        .args([
+            "mcp",
+            "add",
+            "github",
+            "--url",
+            "https://example.com/mcp",
+            "--command",
+            "--",
+            "echo",
+            "hello",
+        ])
+        .assert()
+        .failure()
+        .stderr(contains("unexpected argument '--command' found"));
+
+    let servers = load_global_mcp_servers(codex_home.path()).await?;
+    assert!(servers.is_empty());
+
+    Ok(())
+}