[react-dom] Normalize attribute whitespace during hydration comparison

[Felipeness]

Summary

Related to #35481 — companion to #35978

Browsers normalize tab (\t), newline (\n), and carriage return (\r) characters to spaces when parsing HTML attribute values (HTML spec). React's hydration attribute comparison uses strict string equality, which can produce false mismatch warnings when server and client attribute values differ only by this whitespace normalization.

This change adds a fallback comparison in the hydration path that normalizes \t, \n, and \r to spaces before comparing, matching browser behavior.

What changed

In ReactDOMComponent.js:

• Added NORMALIZE_ATTRIBUTE_WHITESPACE_REGEX and normalizeAttributeValueForComparison() helper
• Modified hydrateAttribute(): after strict equality fails, performs a normalized comparison before reporting a mismatch
• Modified warnForPropDifference(): adds whitespace normalization as an additional comparison step

Context

• This fix targets the DEV warning path — in production, React does not compare individual attributes during hydration (only text content)
• Defensive fix — even if source transforms normalize whitespace (see [compiler] Normalize whitespace in JSX string attributes for builtin tags #35978), this ensures React's hydration comparison is resilient to \t\n\r ↔ space differences from any source
• Preserves existing behavior — strict equality is checked first (fast path), normalization is only a fallback

Relationship to #35978

PR	Layer	What it fixes
#35978	React Compiler codegen	Normalizes whitespace in compiled JSX attribute output
This PR	React DOM hydration	Prevents false mismatch warnings for whitespace-only differences

Either fix independently resolves the reported issue. Together they provide defense in depth.

How did you test this change?

Added 3 test cases to ReactDOMHydrationDiff-test.js:

Test	Scenario
does not warn when attribute values differ only by whitespace normalization	className with \n vs space
does not warn for tab and carriage return normalization	className with \t and \r\n vs spaces
does not warn for title attribute whitespace normalization	title with \n vs space

yarn test packages/react-dom/src/__tests__/ReactDOMHydrationDiff-test.js
# Test Suites: 1 passed, 1 total
# Tests:       40 passed, 40 total (37 existing + 3 new)

[Felipeness]

Closing this PR after internal review. The premise that browsers normalize \t\n\r to spaces in HTML attribute values is incorrect — the HTML5 tokenizer preserves these characters as-is in the attribute value (the "Anything else" rule in the double-quoted attribute value state).

The actual root cause is in the compiler's codegen layer (Babel serialization artifact), which is addressed in #35978. With that fix in place, server and client attribute values are identical, making this hydration-side normalization unnecessary.

Additionally, this PR had issues:

• Suppressed potentially real mismatches for attributes like title where \n renders as a visible line break in tooltips
• Added permanent dev-mode overhead for a problem that should be fixed at the source
• Test cases simulated the mismatch by passing different strings, not by reproducing the actual compiler bug