[codex] Remove string-keyed MCP tool maps (#21454)

## Summary

This PR removes the synthetic `HashMap<String, ToolInfo>` keys from MCP
tool discovery. `McpConnectionManager::list_all_tools()` now returns
normalized `Vec<ToolInfo>`, and downstream code derives identity from
`ToolInfo::canonical_tool_name()`.

The motivation is to keep model-visible tool identity on
`ToolName`/`ToolInfo` instead of parallel string map keys, so future
namespace changes do not have to preserve otherwise-unused lookup keys.

## Changes

- Rename the MCP normalization path from `qualify_tools` to
`normalize_tools_for_model` and return tool values directly.
- Flow MCP tool lists through connectors, plugin injection, router/spec
building, code mode, and tool search as vectors/slices.
- Keep direct/deferred subtraction local to `mcp_tool_exposure`, using
`ToolName` values.
- Update tests to compare `ToolName` instances where MCP identity
matters.

## Validation

- `cargo test -p codex-mcp test_normalize_tools`
- `cargo test -p codex-core mcp_tool_exposure`
- `cargo test -p codex-core
direct_mcp_tools_register_namespaced_handlers`
- `cargo test -p codex-core
search_tool_registers_namespaced_mcp_tool_aliases`
- `just fix -p codex-mcp`
- `just fix -p codex-core`

codex-rs/core/src/plugins/injection.rs

@@ -1,5 +1,4 @@
 use std::collections::BTreeSet;
-use std::collections::HashMap;
 
 use codex_connectors::metadata::connector_display_label;
 use codex_protocol::models::ResponseItem;
@@ -14,7 +13,7 @@ use codex_mcp::ToolInfo;
 
 pub(crate) fn build_plugin_injections(
     mentioned_plugins: &[PluginCapabilitySummary],
-    mcp_tools: &HashMap<String, ToolInfo>,
+    mcp_tools: &[ToolInfo],
     available_connectors: &[connectors::AppInfo],
 ) -> Vec<ResponseItem> {
     if mentioned_plugins.is_empty() {
@@ -27,7 +26,7 @@ pub(crate) fn build_plugin_injections(
         .iter()
         .filter_map(|plugin| {
             let available_mcp_servers = mcp_tools
-                .values()
+                .iter()
                 .filter(|tool| {
                     tool.server_name != CODEX_APPS_MCP_SERVER_NAME
                         && tool