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/core/src/session/mod.rs

@@ -64,7 +64,6 @@ use codex_login::auth_env_telemetry::collect_auth_env_telemetry;
 use codex_login::default_client::originator;
 use codex_mcp::McpConnectionManager;
 use codex_mcp::McpRuntimeEnvironment;
-use codex_mcp::ToolInfo;
 use codex_mcp::codex_apps_tools_cache_key;
 use codex_models_manager::manager::RefreshStrategy;
 use codex_models_manager::manager::SharedModelsManager;
@@ -75,7 +74,6 @@ use codex_otel::current_span_trace_id;
 use codex_otel::current_span_w3c_trace_context;
 use codex_otel::set_parent_from_w3c_trace_context;
 use codex_protocol::ThreadId;
-use codex_protocol::ToolName;
 use codex_protocol::account::PlanType as AccountPlanType;
 use codex_protocol::approvals::ElicitationRequestEvent;
 use codex_protocol::approvals::ExecPolicyAmendment;