[compiler] Normalize whitespace in JSX string attributes for builtin tags

[Felipeness]

Summary

Fixes #35481

Root cause

The Babel JSX transform (@babel/plugin-transform-react-jsx) normalizes newlines followed by whitespace in plain JSX string attributes:

// @babel/plugin-transform-react-jsx/lib/create-plugin.js:260-263
if (isStringLiteral(value) && !isJSXExpressionContainer(attribute.node.value)) {
  value.value = value.value.replace(/\n\s+/g, " ");
  delete value.extra?.raw;
}

This normalization is skipped for JSXExpressionContainer values. When the React Compiler wraps strings in expression containers (because \n matches STRING_REQUIRES_EXPR_CONTAINER_PATTERN in the U+0000-U+001F range), the JSX transform bypasses normalization. The server code (uncompiled) goes through the standard normalization, creating a hydration mismatch.

Fix

Apply the same /\n\s+/g → " " normalization that the JSX transform uses, before the expression container check. This ensures compiled output matches the behavior of non-compiled code. After normalization, strings that no longer contain control characters avoid unnecessary expression container wrapping entirely.

Scope

• Builtin HTML elements only (div, span, etc.) — component props are preserved unchanged
• fbt operands excluded — internationalization strings are not modified
• Uses the exact same regex as @babel/plugin-transform-react-jsx

Before (compiled output)

// Input: className="\n        flex min-h-screen\n        dark:bg-black\n      "
// Compiled: className={"\n        flex min-h-screen\n        dark:bg-black\n      "}
//           ↑ expression container bypasses JSX transform normalization → hydration mismatch

After (compiled output)

// Input: className="\n        flex min-h-screen\n        dark:bg-black\n      "
// Compiled: className=" flex min-h-screen dark:bg-black "
//           ↑ matches JSX transform output, no expression container needed

How did you test this change?

5 fixture tests covering positive and negative scenarios:

Fixture	Scenario	Expected behavior
repro-multiline-classname-hydration	Reporter's exact case: multiline Tailwind className	\n + indentation → single space
repro-multiline-classname-newline-indent	Expression container {"foo\n bar\n baz"}	\n\s+ normalized → "foo bar baz"
repro-multiline-title-attribute	title with \n + indentation	Same normalization as className
repro-multiline-classname-expression	{"foo\nbar"} — \n without trailing whitespace	NOT normalized (no \s+ after \n, matches JSX transform)
repro-multiline-component-prop-preserved	<MyComponent data={"\n"}>	NOT normalized (component, not builtin tag)

yarn snap -p repro-multiline-classname-hydration       # 1 Passed
yarn snap -p repro-multiline-classname-newline-indent   # 1 Passed
yarn snap -p repro-multiline-title-attribute            # 1 Passed
yarn snap -p repro-multiline-classname-expression       # 1 Passed
yarn snap -p repro-multiline-component-prop-preserved   # 1 Passed
yarn workspace babel-plugin-react-compiler lint         # Clean