Simplify MCP tool handler plumbing (#21595)

## Why
The MCP tool path had accumulated a few core-owned special cases: a
dedicated payload variant, resolver plumbing, a legacy `AfterToolUse`
translation path, and a side channel for parallel-call metadata. That
made `ToolRegistry` and the spec builder know more about MCP than they
needed to.

This change moves MCP-specific execution details back onto `ToolInfo`
and `McpHandler` so `codex-core` can treat MCP calls like normal
function calls while still preserving MCP-specific dispatch and
telemetry behavior where it belongs.

## What changed
- removed `resolve_mcp_tool_info`, `ToolPayload::Mcp`, `ToolKind`, and
the remaining registry-side MCP resolver path
- stored MCP routing metadata directly on `McpHandler` and `ToolInfo`,
including `supports_parallel_tool_calls`
- deleted the legacy `AfterToolUse` consumer in `core`, which removes
the need for handler-specific `after_tool_use_payload` implementations
- switched tool-result telemetry to handler-provided tags and kept
MCP-specific dispatch payload construction inside the handler
- simplified tool spec planning/building by passing `ToolInfo` directly
and dropping the direct/deferred MCP wrapper structs and the
parallel-server side table

## Testing
- `cargo check -p codex-core -p codex-mcp -p codex-otel`
- `cargo test -p codex-core
mcp_parallel_support_uses_exact_payload_server`
- `cargo test -p codex-core
direct_mcp_tools_register_namespaced_handlers`
- `cargo test -p codex-core
search_tool_description_lists_each_mcp_source_once`
- `cargo test -p codex-mcp
list_all_tools_uses_startup_snapshot_while_client_is_pending`
- `just fix -p codex-core -p codex-mcp -p codex-otel`

codex-rs/rollout-trace/src/tool_dispatch.rs

@@ -93,11 +93,6 @@ pub enum ToolDispatchPayload {
         additional_permissions: Option<AdditionalPermissionProfile>,
         justification: Option<String>,
     },
-    Mcp {
-        server: String,
-        tool: String,
-        raw_arguments: String,
-    },
 }
 
 /// Result data returned from a dispatch-level tool call.
@@ -263,14 +258,7 @@ fn requester_fields(
     }
 }
 
-fn dispatched_tool_kind(tool_name: &str, payload: &ToolDispatchPayload) -> ToolCallKind {
-    if let ToolDispatchPayload::Mcp { server, tool, .. } = payload {
-        return ToolCallKind::Mcp {
-            server: server.clone(),
-            tool: tool.clone(),
-        };
-    }
-
+fn dispatched_tool_kind(tool_name: &str, _payload: &ToolDispatchPayload) -> ToolCallKind {
     match tool_name {
         "exec_command" | "local_shell" | "shell" | "shell_command" => ToolCallKind::ExecCommand,
         "write_stdin" => ToolCallKind::WriteStdin,
@@ -291,12 +279,8 @@ fn dispatched_tool_kind(tool_name: &str, payload: &ToolDispatchPayload) -> ToolC
 fn dispatched_tool_label(
     tool_name: &str,
     tool_namespace: Option<&str>,
-    payload: &ToolDispatchPayload,
+    _payload: &ToolDispatchPayload,
 ) -> String {
-    if let ToolDispatchPayload::Mcp { server, tool, .. } = payload {
-        return format!("mcp:{server}:{tool}");
-    }
-
     match tool_namespace {
         Some(namespace) => format!("{namespace}.{tool_name}"),
         None => tool_name.to_string(),
@@ -310,7 +294,6 @@ impl ToolDispatchPayload {
             ToolDispatchPayload::ToolSearch { arguments } => truncate_preview(&arguments.query),
             ToolDispatchPayload::Custom { input } => truncate_preview(input),
             ToolDispatchPayload::LocalShell { command, .. } => truncate_preview(&command.join(" ")),
-            ToolDispatchPayload::Mcp { raw_arguments, .. } => truncate_preview(raw_arguments),
         }
     }
 
@@ -346,16 +329,6 @@ impl ToolDispatchPayload {
                 "additional_permissions": additional_permissions,
                 "justification": justification,
             }),
-            ToolDispatchPayload::Mcp {
-                server,
-                tool,
-                raw_arguments,
-            } => json!({
-                "type": "mcp",
-                "server": server,
-                "tool": tool,
-                "raw_arguments": raw_arguments,
-            }),
         }
     }
 }