fix(geolocation): replace setClient with Lookup-based factory SPI

[heruan]

Summary

• Promotes GeolocationClient to a public SPI and adds a GeolocationClientFactory extension point resolved through com.vaadin.flow.di.Lookup.
• Geolocation queries the factory at construction time and falls back to the built-in browser-backed client when none is registered.
• setClient is retained as a package-private in-JAR seam for flow-server's own tests; external test drivers and native bridges register a factory via META-INF/services.

Why

Follow-up to #24211. The setClient seam (originally package-private) breaks under split-classloader topologies (Quarkus, OSGi, JPMS): the JVM scopes runtime packages by classloader, so a class in flow-server.jar and one in an external JAR with the same package name end up in different runtime packages when their JARs load through different classloaders. Cross-JAR package-private access then throws IllegalAccessError at class definition time. Reproduced locally and in CI on browserless-test PR #51:

java.lang.IllegalAccessError: class com.vaadin.flow.component.geolocation.BrowserlessGeolocationClient
  cannot access its superinterface com.vaadin.flow.component.geolocation.GeolocationClient
  (BrowserlessGeolocationClient is in unnamed module of loader QuarkusClassLoader @34eb5d01;
   GeolocationClient            is in unnamed module of loader QuarkusClassLoader @272778ae)

Lookup is classloader-aware and explicitly designed for this kind of pluggable SPI — it's the canonical Flow pattern (InstantiatorFactory, RoutePathProvider, ResourceProvider, StaticFileHandlerFactory, BrowserLiveReloadAccessor).

Changes

• GeolocationClient → public interface with proper SPI Javadoc (no longer "framework internal").
• GeolocationClientFactory (new) → public interface, resolved via Lookup; documents the use cases (in-memory test drivers; native bridges in Cordova/Electron/Capacitor shells).
• Geolocation constructor calls resolveClient(ui) → looks up a factory via VaadinService.getCurrent() → Lookup.lookup(GeolocationClientFactory.class); falls back to new BrowserGeolocationClient(ui, seed) when no factory is registered.
• setClient stays package-private — flow-server's own tests use it; external code goes through Lookup.
• BrowserGeolocationClient Javadoc updated to mention it's the no-factory default.
• GeolocationClientSeamTest now exercises both the public Lookup-factory path and the in-package setClient seam.

No META-INF/services file is shipped from flow-server: production has no factory and uses the fallback. Implementations live in external JARs (e.g. browserless-test-shared).

Test plan

• mvn test -pl :flow-server -Dtest='*Geolocation*' — 38/38 pass locally.
• No @SuppressWarnings("NullAway") introduced; constructor assigns the client field directly.
• Local end-to-end verification with browserless-test patched to register a factory: Quarkus suite that previously threw IllegalAccessError now passes (junit6 21/21, quarkus 11/11, including QuarkusBrowserlessBaseClassTest and QuarkusUnitSecurityTest that were failing).
• Reviewer to confirm SPI shape (factory vs. direct service) is acceptable for Flow's public API surface.

cc @mcollovati

[github-actions]

Test Results

 1 396 files  ±0   1 396 suites  ±0   1h 15m 56s ⏱️ -40s
10 089 tests +1  10 019 ✅ +1  70 💤 ±0  0 ❌ ±0 
10 564 runs  +1  10 485 ✅ +1  79 💤 ±0  0 ❌ ±0 

Results for commit 5be9d25. ± Comparison against base commit 9ac549e.

♻️ This comment has been updated with latest results.

[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