Make ProviderTokenArgs immutable in Node and Java

The args object is constructed solely by the SDK and only read inside the
consumer callback, so it should not be mutable by consumers.

- Java: make fields final, keep only the all-args constructor, and drop the
  no-arg constructor and the fluent setters. The runtime already builds it via
  new ProviderTokenArgs(providerName, sessionId).
- Node: mark the interface fields readonly.

.NET already uses init-only required properties and Rust passes a shared
reference, so both are already immutable. Go and Python have no idiomatic
readonly equivalent for these DTO shapes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

java/src/main/java/com/github/copilot/rpc/ProviderTokenArgs.java

@@ -17,15 +17,9 @@
 @CopilotExperimental
 public class ProviderTokenArgs {
 
-    private String providerName;
+    private final String providerName;
 
-    private String sessionId;
-
-    /**
-     * Creates an empty argument object.
-     */
-    public ProviderTokenArgs() {
-    }
+    private final String sessionId;
 
     /**
      * Creates argument object for the named provider.
@@ -54,18 +48,6 @@ public String getProviderName() {
         return providerName;
     }
 
-    /**
-     * Sets the name of the BYOK provider needing a token.
-     *
-     * @param providerName
-     *            the provider name
-     * @return this args instance for method chaining
-     */
-    public ProviderTokenArgs setProviderName(String providerName) {
-        this.providerName = providerName;
-        return this;
-    }
-
     /**
      * Gets the id of the session that triggered this token request.
      * <p>
@@ -78,16 +60,4 @@ public ProviderTokenArgs setProviderName(String providerName) {
     public String getSessionId() {
         return sessionId;
     }
-
-    /**
-     * Sets the id of the session that triggered this token request.
-     *
-     * @param sessionId
-     *            the session id
-     * @return this args instance for method chaining
-     */
-    public ProviderTokenArgs setSessionId(String sessionId) {
-        this.sessionId = sessionId;
-        return this;
-    }
 }

nodejs/src/types.ts

@@ -2230,15 +2230,15 @@ export interface ProviderTokenArgs {
      * The callback closes over its own token scope/audience; the runtime is
      * provider-agnostic and forwards only the provider name.
      */
-    providerName: string;
+    readonly providerName: string;
 
     /**
      * Id of the session that triggered this token request. A client-level shared
      * callback registered for many sessions can use this to resolve the owning
      * session (e.g. via the client's session lookup) to scope token acquisition
      * or caching per session.
      */
-    sessionId: string;
+    readonly sessionId: string;
 }
 
 /**