Resolve MCP server registrations through a catalog (#27634)

## Why

MCP servers currently come from user config, local plugins,
compatibility Apps synthesis, and host extensions. Those sources were
composed by mutating a shared map, leaving registration identity,
precedence, removal, and provenance implicit in assembly order.

Before adding executor-owned MCPs, Codex needs one durable resolution
boundary above `McpConnectionManager`. This PR introduces that boundary
while preserving current server configuration, policy, and runtime
behavior. Executor-scoped registrations and explicit policy layers
remain follow-ups.

## What changed

- Add typed `McpServerRegistration` inputs and an immutable
`ResolvedMcpCatalog` in `codex-mcp`.
- Retain each registration's complete `McpServerConfig`, including its
environment binding, while recording its source and provenance.
- Preserve the existing structural precedence between plugin, config,
compatibility, and ordered extension sources.
- Resolve equal-precedence actions by contribution order; provenance IDs
are used only for diagnostics and cannot affect the winner.
- Preserve extension removals and the existing name-scoped `enabled =
false` veto.
- Report same-tier conflicts with every contender and the final catalog
outcome, including whether the winning action registers or removes the
server.
- Require MCP contributors to provide a stable diagnostic identity.
- Derive materialized server maps and plugin ownership from the resolved
catalog.

`McpConnectionManager`, transport startup, tool calls, and resource
routing continue to consume the same effective `McpServerConfig` values.

## Scope

This PR does not add new MCP capabilities or change user-visible
behavior. It does not add executor plugin discovery, thread-scoped
registrations, dynamic refresh generations, or new user/managed policy
semantics.

## Verification

- Added focused catalog coverage for source precedence, complete
configuration preservation, disabled vetoes, plugin ownership,
contribution-order tie breaking, removal outcomes, and conflict
diagnostics.
- Extended hosted Apps coverage for ordered extension removal and
Apps-disabled hosts with and without the hosted extension installed.
- `cargo check -p codex-mcp --tests -p codex-extension-api -p
codex-core`

codex-rs/ext/extension-api/src/contributors.rs

@@ -48,6 +48,9 @@ pub type ExtensionFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
 /// Plugin-owned servers and their provenance continue to be resolved by the
 /// plugin manager until that ownership moves into an extension explicitly.
 pub trait McpServerContributor<C: Sync>: Send + Sync {
+    /// Stable identity used for registration provenance and conflict diagnostics.
+    fn id(&self) -> &'static str;
+
     fn contribute<'a>(&'a self, config: &'a C) -> ExtensionFuture<'a, Vec<McpServerContribution>>;
 }