fix(web): make root->dev-ui redirect reverse-proxy-prefix-aware

[luisSilva1234]

Fixes #1457

Summary

AdkWebServer.addViewControllers() redirects "/" to the root-relative path "/dev-ui". Root-relative sendRedirect() targets are left untouched by Spring's own reverse-proxy support (ForwardedHeaderFilter only applies X-Forwarded-Prefix to context-relative targets, via StringUtils.applyRelativePath, which it skips whenever the target already starts with /). So when this app is deployed behind a path-stripping reverse proxy, the browser follows an unprefixed Location header the proxy has no route for, and the dev UI 404s.

This PR makes the redirect context-relative ("dev-ui") instead, so server.forward-headers-strategy=framework + an X-Forwarded-Prefix header from the proxy can correctly restore the prefix.

Behavior for the non-proxied/local case

No change: RedirectView's contextRelative handling only prepends the context path when the target starts with / (so a context-relative target is passed through as-is), and the servlet container's own relative-URL resolution for sendRedirect still resolves "dev-ui" against the current request path "/" to "/dev-ui".

Test plan

Added AdkWebServerReverseProxyTest, covering both cases with server.forward-headers-strategy=framework enabled:

• Without any forwarded headers: redirect is unchanged ("dev-ui") — confirmed ForwardedHeaderFilter doesn't wrap the response at all when the request carries no Forwarded/X-Forwarded-* headers, so this matches the property being off entirely.
• With X-Forwarded-Prefix: /my-app-prefix: redirect becomes http://.../my-app-prefix/dev-ui, confirming the fix.

Also updated AdkWebServerUITest.rootShouldRedirectToDevUi to expect the new context-relative value (MockHttpServletResponse.sendRedirect stores the location verbatim with no resolution, so this assertion needed to change from "/dev-ui" to "dev-ui").

Verified locally end-to-end (JDK 17, as required by this repo):

mvn -pl dev -am test

→ BUILD SUCCESS, 1776 core tests + all dev module tests (including the new and updated ones) pass, 0 failures.

mvn compile was also run to confirm google-java-format is happy with the new file (auto-formatted cleanly, no manual formatting issues found).

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[hemasekhar-p]

Hi @luisSilva1234, thank you for your contribution and We appreciate you taking the time to submit this pull request. Since this is your first contribution, could you please complete the Google CLA so that we can proceed with the review?