fix(dev-bundle): keep sourcemaps usable when preserving usage stats

[totally-not-ai]

Summary

Dev bundles built with build.sourcemap enabled produced .map files with no sources and no mappings, so the browser could not map the bundle back to the original files. The usage statistics Vite plugin now leaves untouched modules alone and emits a proper sourcemap for the one module it rewrites.

What changed

Behavior change: dev bundles built with build.sourcemap enabled now get usable .map files. This only affects apps that turn sourcemaps on for the dev bundle; nothing else in the build changes, and no public Java API is removed or renamed.

• vite.generated.ts: the vaadin:preserve-usage-stats plugin used to return the code of every module it saw without a sourcemap. The transform hook sees all modules of the bundle, so every module was dropped from its chunk's map. The plugin now returns null for modules it does not touch, which keeps the maps the bundler already has.
• The one module it does rewrite (the vaadin-dev-mode:start comment) is now rewritten with MagicString, so the plugin hands back a real sourcemap for it (magic-string was already a build dependency).
• Added SourceMapTestUtil in flow-test-util with the sourcemap assertions that were previously private to SourceMapsIT, so the production and dev bundle tests share them.
• The dev bundle test app now imports a stand-in for @vaadin/vaadin-usage-statistics and builds with sourcemap: true, so the rewrite branch of the plugin is actually exercised.

Follow-up to #25691, related to #16679.

API Changes

com.vaadin.flow.testutil.SourceMapTestUtil

// Added
public final class SourceMapTestUtil
public static List<String> assertSourceMapUsable(String name, String sourceMap) // asserts a sourcemap has sources, mappings and source contents; returns the source file names

Test summary

#	Status	What the test verifies	Why it matters
1	✅	Every .js.map of a dev bundle built with sourcemap: true has non-empty sources, non-empty mappings, and the contents of each source	This is the exact regression: all dev bundle maps came out empty and unusable
2	✅	The dev bundle maps refer back to the real sources, both a normal module (lit-view.ts) and the rewritten usage statistics module	Proves the untouched modules keep the bundler's map and the rewritten module gets its own
3	✅	The built chunk contains /*! vaadin-dev-mode:start and no longer the original /** vaadin-dev-mode:start	The fix must not break the rewrite itself — the comment still has to survive in the bundle
4	✅	Production bundle maps still point to original sources (existing test, now using the shared helper)	Guards against the refactoring changing the production checks
5	❗ gap	The console.error paths (marker missing, rewritten comment fails the regexp)	Only logging; a regression here is silent but low risk

• DevBundleSourceMapsIT.devBundleSourceMaps_pointToOriginalSources → rows 1, 2
• DevBundleSourceMapsIT.usageStatisticsComment_isRewrittenInTheBundle → row 3
• SourceMapsIT.bundleSourceMapsPointToOriginalSources → row 4 (unchanged assertions, moved into SourceMapTestUtil)

Deliberately untested: the plugin's error logging branches (row 5), and SourceMapTestUtil itself, which is covered indirectly by every test that calls it.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

 1 475 files  +1   1 559 suites  +1   1h 40m 13s ⏱️ -28s
12 241 tests +2  12 173 ✅ +2  68 💤 ±0  0 ❌ ±0 
12 559 runs  +2  12 491 ✅ +2  68 💤 ±0  0 ❌ ±0 

Results for commit 0683090. ± Comparison against base commit aaf20ce.

[totally-not-ai]

Closing as a duplicate of #25708, which has the same change to vite.generated.ts. The SourceMapTestUtil helper and the shared assertions from this PR are being ported there.