fix(lsp): open library classes behind jar: URIs (#1048)

The official Kotlin LSP answers navigation into a dependency with a
`jar:///…/lib.jar!/pkg/Name.class` URI but does not serve its contents through
`workspace/textDocumentContent`, so opening the target failed with "no
language server provides the contents of jar:…", locally and remotely.

`jar:` URIs are now resolved by the backend, where the jars are:
- An entry that is not a class (a sources jar entry, a resource) is read from
  its archive.
- A class is shown as its source from the artifact's `-sources.jar` in the
  Maven repository or the Gradle cache (`Name.java`/`Name.kt`, an inner
  class's outer file, a `FileKt` facade's `File.kt`, a `@file:JvmName`
  file); Go to Declaration then lands on the declaration of the name it was
  invoked on.
- Otherwise the server's `decompile` command is used when it offers one, and
  the error says that neither a source nor a decompiler is available.
- Library tabs name the archive and package (`spring-boot-3.5.16.jar ›
  org.springframework.boot`).

Fixes #982

src/platform/virtualDocs.test.ts

@@ -19,6 +19,15 @@ describe("virtual document URIs", () => {
     expect(virtualName(JDT)).toBe("SpringApplication.class");
     expect(virtualParent(JDT)).toBe("jdt://contents/spring-boot-4.1.1.jar/org.springframework.boot");
     expect(virtualContainer(JDT)).toBe("spring-boot-4.1.1.jar › org.springframework.boot");
+    // `jar:` targets (official Kotlin LSP, sources jars), in both URI forms.
+    for (const prefix of ["jar:///", "jar:file:///"])
+      expect(
+        virtualContainer(
+          `${prefix}home/me/.m2/repository/spring-boot-3.5.16-sources.jar!/org/springframework/boot/SpringApplicationExtensions.kt`,
+        ),
+      ).toBe("spring-boot-3.5.16-sources.jar › org.springframework.boot");
+    expect(virtualContainer("jar:///tmp/my%20libs/a.jar!/Root.class")).toBe("a.jar");
+    expect(virtualName("jar:///tmp/a.jar!/org/x/Y.class")).toBe("Y.class");
   });
 
   it("makes path helpers URI-aware", () => {

src/platform/virtualDocs.ts

@@ -37,6 +37,20 @@ export function virtualParent(uri: string): string {
  */
 export function virtualContainer(uri: string): string {
   const scheme = uri.slice(0, uri.indexOf(":"));
+  // `jar:///…/spring-boot-3.5.16.jar!/org/springframework/boot/X.class` → `spring-boot-3.5.16.jar › org.springframework.boot`
+  const jar = scheme === "jar" ? /^jar:(?:file:)?(?:\/\/)?(.*?)!\/(.*)$/.exec(uri.split(/[?#]/)[0]!) : null;
+  if (jar) {
+    const decode = (s: string) => {
+      try {
+        return decodeURIComponent(s);
+      } catch {
+        return s;
+      }
+    };
+    const archive = decode(jar[1]!.split("/").pop() ?? jar[1]!);
+    const pkg = jar[2]!.split("/").slice(0, -1).map(decode).join(".");
+    return pkg ? `${archive} › ${pkg}` : archive;
+  }
   const rest = uri.replace(/^[^:]+:(\/\/)?/, "").split(/[?#]/)[0]!;
   const segs = rest
     .split("/")