Add getSessionMetadata for O(1) session lookup by ID

Port upstream copilot-sdk PR #899 to Java SDK. Adds a new
getSessionMetadata(sessionId) method to CopilotClient that calls the
session.getMetadata JSON-RPC endpoint for efficient single-session
metadata lookup. Returns null when the session is not found.

Changes:
- CopilotClient: new getSessionMetadata(String) method
- GetSessionMetadataResponse: new response DTO record
- SessionMetadata: updated Javadoc to reference new method
- CopilotSessionTest: new @disabled E2E test (matching upstream skip
  pattern for session lifecycle tests)

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

Author: brunoborges

src/main/java/com/github/copilot/sdk/CopilotClient.java

@@ -22,6 +22,7 @@
 import com.github.copilot.sdk.json.DeleteSessionResponse;
 import com.github.copilot.sdk.json.GetAuthStatusResponse;
 import com.github.copilot.sdk.json.GetLastSessionIdResponse;
+import com.github.copilot.sdk.json.GetSessionMetadataResponse;
 import com.github.copilot.sdk.json.GetModelsResponse;
 import com.github.copilot.sdk.json.GetStatusResponse;
 import com.github.copilot.sdk.json.ListSessionsResponse;
@@ -628,6 +629,34 @@ public CompletableFuture<List<SessionMetadata>> listSessions(SessionListFilter f
         });
     }
 
+    /**
+     * Gets metadata for a specific session by ID.
+     * <p>
+     * This provides an efficient O(1) lookup of a single session's metadata instead
+     * of listing all sessions. Returns {@code null} if the session is not found.
+     *
+     * <h2>Example Usage</h2>
+     *
+     * <pre>{@code
+     * SessionMetadata metadata = client.getSessionMetadata("session-123").get();
+     * if (metadata != null) {
+     * 	System.out.println("Session started at: " + metadata.getStartTime());
+     * }
+     * }</pre>
+     *
+     * @param sessionId
+     *            the ID of the session to look up
+     * @return a future that resolves with the session metadata, or {@code null} if
+     *         not found
+     * @see SessionMetadata
+     * @see #listSessions()
+     */
+    public CompletableFuture<SessionMetadata> getSessionMetadata(String sessionId) {
+        return ensureConnected().thenCompose(connection -> connection.rpc
+                .invoke("session.getMetadata", Map.of("sessionId", sessionId), GetSessionMetadataResponse.class)
+                .thenApply(GetSessionMetadataResponse::session));
+    }
+
     /**
      * Gets the ID of the session currently displayed in the TUI.
      * <p>

src/main/java/com/github/copilot/sdk/json/GetSessionMetadataResponse.java

@@ -0,0 +1,24 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------------------------------------------*/
+
+package com.github.copilot.sdk.json;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+/**
+ * Internal response object from getting session metadata.
+ * <p>
+ * This is a low-level class for JSON-RPC communication containing the metadata
+ * for a specific session, or {@code null} if the session was not found.
+ *
+ * @see com.github.copilot.sdk.CopilotClient#getSessionMetadata(String)
+ * @see SessionMetadata
+ * @since 1.0.0
+ */
+@JsonInclude(JsonInclude.Include.NON_NULL)
+public record GetSessionMetadataResponse(
+        /** The session metadata, or {@code null} if the session was not found. */
+        @JsonProperty("session") SessionMetadata session) {
+}

src/main/java/com/github/copilot/sdk/json/SessionMetadata.java

@@ -11,7 +11,9 @@
  * Metadata about an existing Copilot session.
  * <p>
  * This class represents session information returned when listing available
- * sessions via {@link com.github.copilot.sdk.CopilotClient#listSessions()}. It
+ * sessions via {@link com.github.copilot.sdk.CopilotClient#listSessions()} or
+ * looking up a specific session via
+ * {@link com.github.copilot.sdk.CopilotClient#getSessionMetadata(String)}. It
  * includes timing information, a summary of the conversation, and whether the
  * session is stored remotely.
  *

src/test/java/com/github/copilot/sdk/CopilotSessionTest.java

@@ -7,6 +7,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
 
@@ -19,6 +20,7 @@
 
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import com.github.copilot.sdk.events.AbstractSessionEvent;
@@ -693,6 +695,48 @@ void testShouldDeleteSession() throws Exception {
         }
     }
 
+    /**
+     * Verifies that session metadata can be retrieved by ID.
+     * <p>
+     * TODO: Re-enable once test harness CAPI proxy supports this test's session
+     * lifecycle.
+     *
+     * @see Snapshot: session/should_get_session_metadata
+     */
+    @Disabled("Needs test harness CAPI proxy support")
+    @Test
+    void testShouldGetSessionMetadata() throws Exception {
+        ctx.configureForTest("session", "should_get_session_metadata");
+
+        try (CopilotClient client = ctx.createClient()) {
+            CopilotSession session = client
+                    .createSession(new SessionConfig().setOnPermissionRequest(PermissionHandler.APPROVE_ALL)).get();
+
+            session.sendAndWait(new MessageOptions().setPrompt("Say hello")).get(60, TimeUnit.SECONDS);
+
+            // Small delay to ensure session file is written to disk
+            Thread.sleep(200);
+
+            // Get metadata for the session we just created
+            var metadata = client.getSessionMetadata(session.getSessionId()).get(30, TimeUnit.SECONDS);
+            assertNotNull(metadata);
+            assertEquals(session.getSessionId(), metadata.getSessionId());
+            assertNotNull(metadata.getStartTime());
+            assertNotNull(metadata.getModifiedTime());
+
+            // Verify context field
+            if (metadata.getContext() != null) {
+                assertNotNull(metadata.getContext().getCwd());
+            }
+
+            // Verify non-existent session returns null
+            var notFound = client.getSessionMetadata("non-existent-session-id").get(30, TimeUnit.SECONDS);
+            assertNull(notFound);
+
+            session.close();
+        }
+    }
+
     /**
      * Verifies that sessions can be created with custom tools.
      *