New checkbox setting to allow short list in tooltip

[cdavalos7]

Motivation for features / changes

We wrapped a previous functionality to reduce redundancy of data in the tooltip for Scalar Chart into a checkbox so users can choose either to display the tooltip with the full list or short list for exploring data.

Technical description of changes

• Added a new limitTooltipRows setting. As redux setting like the other controls in panel settings: action → reducer → selector → container → presentational component input.
• New checkbox added to the metrics settings panel; defaults to unchecked box (full tooltip shown).
• When on, the scalar card tooltip caps at MAX_TOOLTIP_ITEMS rows and shows a "N additional items" legend for the overflow.
• Setting persists to localStorage, so it sticks across reloads, same pattern as the other metrics settings.
• Added unit tests covering the on/off behavior of the tooltip and persistance.

Detailed steps to verify changes work correctly (as executed by you)

• Ran the full frontend Karma suite bazel test ... tensorboard/webapp:karma_test_chromium-local
• Verified the new functionality tests
• Verified the new persistence tests

Screenshots of UI changes (or N/A)