fix: emit servlet-relative href for AppShell @StyleSheet (#24233) (CP: 25.1)

[vaadin-bot]

This PR cherry-picks changes from the original PR #24233 to branch 25.1.

Original PR description

AppShellRegistry.resolveStyleSheetHref expanded context://-prefixed @Stylesheet values server-side using request.getContextPath() + "/", producing absolute server paths like that get baked into index.html. This breaks behind reverse proxies that don't preserve the servlet container's context path in the public URL: the server emits /foo/styles.css but the browser fetches it from the public host where /foo/ doesn't exist.

Use service.getContextRootRelativePath(request) instead — the same servlet-relative path (./, ../, etc.) that the bootstrap callback populates into CONTEXT_ROOT_URL for the UIDL path. The resulting href is resolved by the browser against , which Vaadin sets from the actual request URL (honoring X-Forwarded-* headers).

This brings AppShell-level @Stylesheet resolution in line with the component-level UIDL path, which already used the relative form via the client-side URIResolver.

Test fixtures updated to reflect the new servlet-relative hrefs. AppShellRegistryAuraAutoLoadTest had a Mockito mock that returned null for getContextRootRelativePath; it now stubs "./".

Related to #24218.

[vaadin-bot]

This PR is eligible for auto-merging policy, so it has been approved automatically. If there are pending conditions, auto merge (with 'squash' method) has been enabled for this PR [Message is sent from bot]

[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

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[github-actions]

Test Results

 1 387 files  ±0   1 387 suites  ±0   1h 19m 30s ⏱️ +48s
10 017 tests +1   9 947 ✅ +1  70 💤 ±0  0 ❌ ±0 
10 490 runs  +1  10 411 ✅ +1  79 💤 ±0  0 ❌ ±0 

Results for commit 728a3a7. ± Comparison against base commit f35fe9c.