input: Make rows set the height of a plain Textarea again

[Bombatomica64]

Closes #3325

Description

TextareaState::rows(n) had no effect on a plain (non-auto-grow) Textarea: it was always one line tall unless the app gave it an explicit height. The Textarea story compensates with .h(px(320.)) next to .rows(10).

Root cause: LayoutMode::update_auto_grow runs for every multi-line input (on layout changes and edits) and calls set_rows(wrapped_line_count). For PlainText and CodeEditor that overwrote the configured rows with the content's line count. A rows-based minimum height would then have grown the input to fit its whole content. Since #1191 the non-auto-grow branch in TextElement::request_layout has used a one-line minimum instead, and rows was effectively ignored.

This PR:

• makes update_auto_grow a no-op outside auto-grow mode, so a configured rows survives layout and edits;
• uses the same rows * line_height minimum height for every multi-line mode again (auto-grow keeps its max_rows cap);
• fixes the stale rows doc comment ("default: 2" → 1).

Behaviour change to note: a default Textarea is unchanged (1 row), but a code editor's default rows: 2 now applies, so an editor in a container shorter than two lines grows to two lines. I found no test or story that depends on that. Happy to keep editors at one line if you prefer.

Screenshot

Screen recordings on Android (Redroid, dark theme) of opening a screen with a .rows(5) Textarea (4 lines of content) and a .rows(3) Textarea with inline tokens, Kit 0.7.0 with and without this diff.

Before

pr3330-before.mp4

After

pr3330-after.mp4

Before: both Textareas are one line tall and the rest is clipped. After: they are five and three rows tall.
Builds: demo-pr3330.

How to Test

• New rows_set_the_minimum_height_of_a_plain_textarea in crates/kit/tests/input/textarea.rs renders rows(1), rows(2) and rows(5) Textareas in auto-height containers and checks that five rows add exactly four times what the second row adds. It fails on main.
• Restoring the minimum height alone (without the update_auto_grow gate) breaks 6 existing tests: fixed-viewport navigation, page movement, decoration clipping, search reveal and touch-menu scrolling. That breakage exposed the root cause, and all six pass with this change.
• cargo test -p gpui-kit --features test-support,component --tests: all pass (input: 161).
• cargo test -p gpui-base --features test-support --lib: 1244 passed.
• cargo test -p gpui-component --features test-support --lib: 578 passed.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes. (Not run: no desktop session here; see How to Test.)
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific) (not platform-specific)

Thanks for taking the time to review this.

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[Bombatomica64]

I've added before/after screen recordings to the Screenshot section (Android, dark theme, Kit 0.7.0 with and without this diff).

Before

pr3330-before.mp4

After

pr3330-after.mp4

The first Textarea is .rows(5) and the inline-tokens one is .rows(3).