text: Break inline flow at hard newlines (#2676)

Closes #2673

## Description

Fix a panic when a Markdown paragraph contains a newline before an
inline HTML image, for example:

```markdown
Hi
[<img src="whatever">](https://google.com/)
```

`InlineFlow` previously passed a text range containing a hard line break
to GPUI's single-line text shaper. GPUI's line wrapper skips `\n` during
width calculation but does not split the range, so the newline later
reached `shape_line`, which rejects multiline input and panicked.

This change splits inline-flow content at hard newlines first, then
applies soft wrapping to each hard line independently. It also adds a
regression test using the issue's Markdown structure.

AI assistance: Codex assisted with root-cause analysis, implementation,
testing, and PR wording. The final changes were reviewed and tested
manually.

## Before / After

The same Markdown was tested in the Markdown Editor before and after
this change:

```markdown
Hi
[<img src="https://github.githubassets.com/favicons/favicon.svg">](https://google.com/)
```

### Before

The application panics while laying out the Markdown because a newline
reaches the text shaper:

<img width="1947" height="344" alt="Before: the Markdown example panics
with text argument should not contain newlines"
src="https://github.com/user-attachments/assets/947e1614-1445-44b6-a11c-527a73bd2087"
/>

### After

The Markdown renders successfully, with the linked inline image
appearing on the line after `Hi`:

<img width="2727" height="1617" alt="After: the Markdown Editor renders
the text and linked inline image"
src="https://github.com/user-attachments/assets/69f0bcef-c11a-4e5b-b7aa-268cebd5ab17"
/>

## How to Test

```bash
cargo test -p gpui-component inline_html_image_after_newline_does_not_panic
cargo test -p gpui-component
cargo fmt --package gpui-component -- --check
```

Results:

- Regression test passes.
- All 407 `gpui-component` tests pass.
- Formatting check passes.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI-generated code is
accurate.
- [X] Passed `cargo run` for story tests related to the changes.
- [x] Platform-specific performance testing is not applicable to this
fix.