text: Keep a line with inline code as tall as a plain line

[huacnlee]

Fixes #3162

Description

In a Markdown TextView, a line containing inline code was taller than a plain line, so a tight list with one code item was unevenly spaced (the code item looked pushed down). A line with inline code is laid out by InlineFlow, a plain line by GPUI's text layout, and they disagreed on the line height in two ways:

• Per-run leading. Every text run was centered in its own line-height box and that box was merged into the line. The inline-code run (mono font at 0.875) has different ascent/descent proportions from the body font, so the merged line grew, on one side only. The body strut was also stretched to the font's ascent + descent when that exceeds the line height, which a plain line never does.
• Rounding. The flow took its line height from Window::line_height, which rounds to a whole logical pixel before snapping to device pixels. StyledText does not round first, so at fractional scale factors (e.g. 1.6: 25.888 → 26 → 26.25 instead of 25.625) the flow line was one device pixel taller.

Changes in crates/base/src/text/inline_flow.rs:

• Resolve the flow's line height exactly as StyledText does.
• Keep the body line box at the line height; text runs join the line by their glyph box and may overflow it as far as body glyphs do.
• When positioning, give each text run's box equal leading on both sides as far as the line allows, so its glyphs stay on the line's baseline and a body run gets exactly the line box. The code background moves with it.

Measured in the real app at scale 1.6 with the issue's list (plus a CJK list), gaps between bullets in device pixels:

	gaps
Before	41, 43, 41, 41, 44
After	41, 41, 41, 41, 41

No public API changes.

How to Test

• cargo test -p gpui-base --lib; new regressions inline_code_line_is_as_tall_as_a_plain_line (scale 1.6/2, zoom 1/1.25) and inline_code_line_is_as_tall_as_a_plain_line_when_glyphs_overflow_it fail before this change.
• Render - plain item one\n- item with \inline code`\n- plain item three` on a fractional-scale display; the items are evenly spaced.

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