[EuiButton][forms][EuiListGroupItem] Control radius (8px)

[ryankeairns]

Part of #9893

Depends on: #9894 (radii tokens) — #9903 adds border.radius.control (8px). Prefer basing this work on #9903 / after it merges; don’t invent a temporary 8px.

Goal

Align interactive controls on the shared control radius (8px) so buttons, form inputs, and list items sit on one corner language.

Decisions (locked)

• border.radius.control (8px) — no full pill
• Keep flat fill on buttons; no elevation/shadow on buttons

Scope

Already spiked (#9905)

• EuiButton / Empty / Icon → euiTheme.border.radius.control
• EuiButtonGroup → border.radius.control

Design feedback (still to wire)

• Form input controls (EuiFormControlLayout / text fields and related inputs) → border.radius.control
• EuiListGroupItem → border.radius.control

Out of scope / later

• One size of buttons/inputs (small) — epic open decision ([Draft][Epic] Modernize Kibana visual language — EUI tokens & component defaults #9893); not required here
• Quieter form edges (color / faux border) — [HOLD][EuiFormControlLayout] Quieter borders #9897 (HOLD) / Token updates: surfaces, radii, borders, semantic spacing #9894
• Panel-tier radii (CallOut, Banner, MarkdownEditor, Panel) — [EuiPanel] Flat by default, softer corners, simpler padding #9895

Implementation

• Spike: [EuiButton] Spike: control radius for Button/Empty/Icon/ButtonGroup #9905