Remove MCP prefix from model-visible tool namespaces

[sayan-oai]

Summary

• expose MCP tools to the model without the internal mcp__ prefix
• preserve flattened connector namespaces such as codex_apps__calendar.create_event
• keep collision handling and historical missing-tool placeholders aligned with the new names

Testing

• cargo test -p codex-tools mcp_namespace_colliding_with_dynamic_namespace_is_skipped
• cargo test -p codex-core tools::spec::tests::search_tool_registers_namespaced_mcp_tool_aliases -- --nocapture