fix(ai): preserve host rendering and metadata in grouped tools

[tiensonqin]

Enabling transcript tool grouping bypassed the host renderer, row metadata and styling. Route group children through the same host-aware body selection as ordinary items, retaining the built-in fallback. Group children use non-interactive keyed columns; item data attributes update without remounting their bodies.

Four mounted regressions fail before the fix and pass afterward, covering custom/default rendering, streamed content and metadata, keyed child retention, disclosure and disposal. The complete 147-case native test_lui target and Melange @web build pass; changed functions pass the 64-line AST check. No Dune changes, warning suppression, unsafe casts or vendoring. Actual Mapre browser acceptance follows installed-package review.

The new decision validates individually. The full document check reports two pre-existing legacy document paths; those unrelated files are unchanged.