text: prevent inline-code fragments from wrapping twice

[wabzqem]

Description

Inline code can wrap its final character onto a second line while its background remains on the first line. This reproduces in example-markdown at 100% zoom with asdfwWwwWMMMdmd.

InlineFlow already shapes, wraps, and positions each fragment, but its nested text element can wrap it again when fractional-width rounding leaves a slightly narrower constraint. Apply whitespace_nowrap() to the existing fragment container so wrapping remains owned by InlineFlow. Long code still wraps through the parent flow; font sizes and padding are unchanged.

Add a full-Markdown regression that inspects actual painted output rather than caret bounds, which can miss the final-character wrap. The test fails without the production fix and passes with it.

Screenshot

(before above, after below)

How to Test

Run cargo run -p example-markdown and enter `asdfwWwwWMMMdmd`. The final d should remain with the rest of the token.

Passed:

• cargo fmt --check
• cargo test --locked -p gpui-base --lib — 889 tests
• cargo clippy --locked -p gpui-base --all-targets --all-features -- -D warnings

Checklist

• Read and followed the contributing guidelines.
• Reviewed the changes, including generated code, for accuracy.
• Interactive story validation/screenshot.
• Windows/Linux validation (not run).