perf(java): reuse usage search results and skip covered folders when widening (#2278)

Find Usages and usage counts searched again on every request: about two
seconds each for a common type such as String.

- The Java usages extension (1.12.0) keeps answers by search target until a
  Java element, the classpath or an unsaved edit changes, and requests for
  the same target share one search.
- Widening checks up to 5000 candidates against the server's analysis scope
  before applying the 300-file limit, so names mentioned in many covered
  files no longer report "too many files".
- Candidate scans skip the source folders the server covers fully
  (openidea.workspace.coveredRoots) instead of reading every file.
- Commands a server registers after initializing are added to its commands.

java/java-usages/MANIFEST.MF

@@ -2,7 +2,7 @@ Manifest-Version: 1.0
 Bundle-ManifestVersion: 2
 Bundle-Name: OpenIDEA Java usages
 Bundle-SymbolicName: ai.openidea.usages;singleton:=true
-Bundle-Version: 1.9.0
+Bundle-Version: 1.12.0
 Bundle-RequiredExecutionEnvironment: JavaSE-21
 Require-Bundle: org.eclipse.jdt.ls.core,org.eclipse.jdt.core,
  org.eclipse.jdt.core.manipulation,org.eclipse.core.runtime,