Represent MCP authentication with an enum (#29924)

## Why

MCP authentication has distinct OAuth and ChatGPT-session flows.
Representing that choice as `use_chatgpt_auth` makes one flow implicit
and allows the configuration model to express the distinction only
through a boolean.

ChatGPT credential forwarding also needs a first-party trust boundary. A
configurable `chatgpt_base_url` controls routing, but must not grant an
MCP server permission to receive session credentials.

This change builds on #29733, where the boolean was introduced.

## What changed

- Replace `use_chatgpt_auth` with an `auth` field backed by the
exhaustive `McpServerAuth` enum.
- Support `auth = "oauth"` and `auth = "chatgpt"`, with OAuth remaining
the default.
- Trust only the origin derived from the existing hardcoded
`CHATGPT_CODEX_BASE_URL` when granting ChatGPT auth to an MCP server.
- Keep configured bearer tokens and authorization headers ahead of the
selected authentication flow.
- Update config writers, schema output, fixtures, and integration-test
setup to use the enum.

## Verification

Integration coverage exercises the complete streamable HTTP startup path
in two independent configurations:

- A directly constructed MCP configuration verifies that matching an
overridden `chatgpt_base_url` does not grant ChatGPT auth.
- A persisted `config.toml` containing an attacker-controlled
`chatgpt_base_url` and `auth = "chatgpt"` verifies the same boundary
through normal config parsing.

Both tests complete MCP initialization and tool listing and assert that
the full captured request sequence contains no authorization headers.
Separate integration coverage verifies that configured authorization
takes precedence over ChatGPT auth.

codex-rs/cli/src/mcp_cmd.rs

@@ -345,7 +345,7 @@ async fn run_add(config_overrides: &CliConfigOverrides, add_args: AddArgs) -> Re
     };
 
     let new_entry = McpServerConfig {
-        use_chatgpt_auth: false,
+        auth: Default::default(),
         transport: transport.clone(),
         environment_id: codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID.to_string(),
         enabled: true,