fix(scale.time): respect per-unit displayFormat in getLabelForValue

[xianjianlf2]

Summary

TimeScale#getLabelForValue always formatted the value with the datetime displayFormat, ignoring the displayFormat configured for the currently resolved time unit. As reported in #12128 (with a pointer to scale.time.js:502), a value returned by getLabelForValue therefore did not match the format the user configured for the active unit.

Root cause

return adapter.format(value, timeOpts.displayFormats.datetime);

The datetime format key was hardcoded, so per-unit displayFormats (e.g. day, hour) were never used.

Fix

Format the value with the displayFormat of the currently resolved unit, falling back to datetime when no per-unit format is configured. tooltipFormat keeps precedence, exactly as before.

const displayFormats = timeOpts.displayFormats;
return adapter.format(value, displayFormats[this._unit] || displayFormats.datetime);

Tests

• Added a unit spec asserting getLabelForValue uses the current unit's displayFormat, and that tooltipFormat still wins.
• Three existing specs asserted the old, buggy output (the hardcoded datetime format) and were corrected to the format of the active unit. These assertions encoded the exact behaviour reported in getLabelForValue returns the wrong format in time scale #12128.

Fixes #12128