feat: make built-in MCPs first-class runtime servers (#21356)

## DISCLAIMER
This is experimental and no production service must rely on this

## Why

Built-in MCPs are product-owned runtime capabilities, but they were
previously flattened into the same config-backed stdio path as
user-configured servers. That made them depend on a hidden `codex
builtin-mcp` re-exec path, exposed them through config-oriented CLI
flows, and erased distinctions the runtime needs to preserve—most
notably whether an MCP call should count as external context for
memory-mode pollution.

## What changed

- Model product-owned built-ins separately from config-backed MCP
servers via `BuiltinMcpServer` and `EffectiveMcpServer`.
- Launch built-ins in process through a reusable async transport instead
of the hidden `builtin-mcp` stdio subcommand.
- Keep config-oriented CLI operations such as `codex mcp
list/get/login/logout` scoped to configured servers, while merging
built-ins only into the effective runtime server set.
- Retain server metadata after launch so parallel-tool support and
context classification come from the live server set; built-in
`memories` is now classified as local Codex state rather than external
context.

## Test plan

- `cargo test -p codex-mcp`
- `cargo test -p codex-core --test suite
builtin_memories_mcp_call_does_not_mark_thread_memory_mode_polluted_when_configured`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/Cargo.lock

@@ -2131,10 +2131,10 @@ name = "codex-builtin-mcps"
 version = "0.0.0"
 dependencies = [
  "anyhow",
- "codex-config",
  "codex-memories-mcp",
  "codex-utils-absolute-path",
  "pretty_assertions",
+ "tokio",
 ]
 
 [[package]]
@@ -2182,7 +2182,6 @@ dependencies = [
  "codex-app-server-protocol",
  "codex-app-server-test-client",
  "codex-arg0",
- "codex-builtin-mcps",
  "codex-chatgpt",
  "codex-cloud-tasks",
  "codex-config",