Harden RichContentFormatter sanitization + add a comprehensive test suite

[jkmassel]

Summary

Stacked on #25833, which fixes the String.count/UTF-16 NSRange bug, hardens parseValueForAttribute against a missing closing quote, and migrates the RichContentFormatter tests to Swift Testing. This PR adds the remaining sanitization fixes the tests + a follow-up audit exposed, plus a comprehensive sanitization test suite.

Base is jkmassel/richcontentformatter-utf16-range (#25833) → which stacks on #25832 → trunk. Retargets up the stack as each parent merges.

Fixes (on top of #25833)

• Unclosed <script>/<style> are now stripped through end of input, not only closed pairs (fail-closed).
• Single-quoted inline styles are stripped, and style= now requires a whitespace boundary so it no longer matches inside a longer name like data-style / data-mce-style.
• formatVideoTags detects the controls attribute as a word-boundaried, case-insensitive token — so a value/name containing controls (poster="…/controls.jpg", class="…-controls", data-controls, controlslist) no longer leaves a video uncontrolled, and CONTROLS isn't duplicated; the <video> match is tightened against <videoxyz>, and controls is inserted in place, preserving the tag's casing.
• parseValueForAttribute matches the attribute name on a word boundary, so "rc" no longer returns src's value. (Upgrades Fix grapheme-vs-UTF-16 range bug in RichContentFormatter #25833's crash guard to a value-capturing regex, which also keeps its no-crash guarantee.)
• resizeGalleryImageURL anchors the src rewrite to the whole src="…" token, so it can't also rewrite the same URL where it appears in srcset.

Tests

A ~75-case parameterized Swift Testing suite (RichContentFormatterSanitizationTests) covering every sanitization method — quote/whitespace/case variants, multiple matches, empty input, no-ops — plus an iOS srcset-preservation regression test. It coexists with #25833's migrated RichContentFormatterTests; all suites run green together (39 tests / 10 suites on the macOS host).

Owned by #25833 (not here)

The String.count→UTF-16 NSRange fix, the removeTrailingBreakTags offsetBy→Range(_:in:) fix, the parseValueForAttribute missing-quote guard, and the Swift Testing test migration.

Known limitation (not addressed)

This is not an XSS sanitizer: event-handler attributes (onerror/onclick), javascript:/data: URLs, and <iframe>/<object> pass through — a best-effort regex limitation, relevant only if output reaches a JS-enabled WKWebView. A follow-up rewrite on SwiftSoup (already a WordPressShared dependency) would dissolve the whole regex-fragility class; this suite is the behavioral contract for it.

Test plan

• swift test --filter RichContentFormatter (macOS host) — 39 tests / 10 suites pass (Fix grapheme-vs-UTF-16 range bug in RichContentFormatter #25833's migrated suite + this suite), no known issues, no crash.
• swift-format stable; SwiftLint clean on all changed files.
• CI green (incl. the iOS-hosted KeystoneTests gallery tests that exercise resizeGalleryImageURL).

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	34347
	Version	PR #25985
	Bundle ID	org.wordpress.alpha
	Commit	76e1945
	Installation URL	6u2crami4tctg

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	34347
	Version	PR #25985
	Bundle ID	com.jetpack.alpha
	Commit	76e1945
	Installation URL	2npqogdffqus8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.