Represent MCP authentication with an enum

[aibrahim-oai]

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.

[aibrahim-oai]

This change is part of the following stack:

• Allow ChatGPT-hosted MCP servers to use session auth #29733
  • Represent MCP authentication with an enum #29924 ◀

_{Change managed by git-spice.}