Fix format state reporting relative em font size for heading elements

[Copilot]

When cursor/selection is inside a heading element (h1–h6), getFormatState() returns the raw relative value (e.g. 1.5em for h2) instead of an absolute pt size, because the heading's default style is stored verbatim in the paragraph decorator format and px2Pt only converts px units.

Root cause

fontSizeFormatHandler stores the raw defaultStyle.fontSize value (e.g. '1.5em' from defaultContentModelFormatMap) without normalization when there is no inline element.style.fontSize. In retrieveModelFormatState, this relative value from the decorator overrides the container's resolved absolute font size in currentFormat, and then passes through px2Pt unchanged.

Fix

retrieveModelFormatState.ts — Before the final px2Pt call, resolve any remaining relative em/% font size against the container's computed font size (obtained from domHelper.getContainerFormat()):

formatState.fontSize = '1.5em'
containerFormat.fontSize = '16px'
→ 1.5 × 16 = 24px → 18pt  ✓

If no domHelper is available (legacy adapter path, unit tests without a DOM), the value is left as-is — no regression.

retrieveModelFormatStateTest.ts — Three new cases:

• em decorator format + domHelper → resolves to absolute pt (1.5em × 16px = 18pt)
• em decorator format, no domHelper → graceful fallback, value unchanged
• % decorator format + domHelper → resolves to absolute pt (150% × 16px = 18pt)

• Fixes format state shows fontSize:1.5em in h2 element #2700