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/otel/src/events/session_telemetry.rs

@@ -65,6 +65,12 @@ const RESPONSES_API_ENGINE_SERVICE_TTFT_FIELD: &str = "engine_service_ttft_total
 const RESPONSES_API_ENGINE_IAPI_TBT_FIELD: &str = "engine_iapi_tbt_across_engine_calls_ms";
 const RESPONSES_API_ENGINE_SERVICE_TBT_FIELD: &str = "engine_service_tbt_across_engine_calls_ms";
 
+fn trace_field_value<'a>(fields: &'a [(&str, &str)], key: &str) -> Option<&'a str> {
+    fields
+        .iter()
+        .find_map(|(field_key, value)| (*field_key == key).then_some(*value))
+}
+
 #[derive(Debug, Clone, Default, PartialEq, Eq)]
 pub struct AuthEnvTelemetryMetadata {
     pub openai_api_key_env_present: bool,
@@ -943,8 +949,7 @@ impl SessionTelemetry {
         call_id: &str,
         arguments: &str,
         extra_tags: &[(&str, &str)],
-        mcp_server: Option<&str>,
-        mcp_server_origin: Option<&str>,
+        extra_trace_fields: &[(&str, &str)],
         f: F,
     ) -> Result<(String, bool), E>
     where
@@ -969,8 +974,7 @@ impl SessionTelemetry {
             success,
             output.as_ref(),
             extra_tags,
-            mcp_server,
-            mcp_server_origin,
+            extra_trace_fields,
         );
 
         result
@@ -1010,8 +1014,7 @@ impl SessionTelemetry {
         success: bool,
         output: &str,
         extra_tags: &[(&str, &str)],
-        mcp_server: Option<&str>,
-        mcp_server_origin: Option<&str>,
+        extra_trace_fields: &[(&str, &str)],
     ) {
         let success_str = if success { "true" } else { "false" };
         let mut tags = Vec::with_capacity(2 + extra_tags.len());
@@ -1020,8 +1023,9 @@ impl SessionTelemetry {
         tags.extend_from_slice(extra_tags);
         self.counter(TOOL_CALL_COUNT_METRIC, /*inc*/ 1, &tags);
         self.record_duration(TOOL_CALL_DURATION_METRIC, duration, &tags);
-        let mcp_server = mcp_server.unwrap_or("");
-        let mcp_server_origin = mcp_server_origin.unwrap_or("");
+        let mcp_server = trace_field_value(extra_trace_fields, "mcp_server").unwrap_or("");
+        let mcp_server_origin =
+            trace_field_value(extra_trace_fields, "mcp_server_origin").unwrap_or("");
         log_event!(
             self,
             event.name = "codex.tool_result",