Previously ReadToolGroupComponent completely ignored the expanded
state and discarded result content, making read tool output invisible
in the TUI (only showed a one-line path summary with no file content).
Changes:
- Store expanded state in #expanded field (was discarded with _ prefix)
- Store result content text in ReadEntry.contentText (was thrown away)
- Show a small code preview (3 lines) by default in collapsed mode
- Show full file content when expanded via Ctrl+O
- Uses renderCodeCell with PREVIEW_LIMITS.OUTPUT_COLLAPSED for
consistent truncation with "... N more lines (Ctrl+O for more)" hint
This brings read tool output in line with bash and grep tools which
already show previews when collapsed.
