fix(session-ui): use the filled chevron for Code Mode call rows - #55947

Code Mode call rows inside an expanded Execute tool used a › text glyph as their marker, while the Execute header next to them uses the filled triangle (fill-triangle-down) from Collapsible.Arrow. This swaps the glyph for the same filled icon so the two levels match.

• The leading marker points right and turns down when the call row is expanded, like the header arrow.
• The trailing arrow on failed calls uses the same icon and rotation (it previously rotated a › glyph 90°).
• The warning marker for failed calls is unchanged.

Verified in the Current Session/Terminal Work → Execute failed story in light and dark: collapsed, expanded successful, and expanded failed rows. bun run typecheck passes in packages/session-ui.

Requested by: @Hona (Hona via Slack)