test(branch): log in with EMAIL/PASSWORD and manage own environments in BranchAPITest

BranchAPITest used TestClient.getStack(), which sends the management token.
Branch create answered 401 (error 161) with it, and the test assumed the
'development' and 'production' environments exist on the stack.

The class now:
- logs in with EMAIL / PASSWORD and uses the apiKey header only;
- creates two environments whose names are unique to the run, and deletes
  them, with every created branch (force=true), and logs out in @afterall;
- polls fetch until a created branch is ready (bounded, 60s, 2s steps);
- treats a rejected create as no branch when fetch answers 422 with
  error_code 905, the same answer as a missing branch;
- adds TC-23 (create without publish_config) and TC-24 (environment name in
  a different case gets 422).

Without EMAIL, PASSWORD or apiKey in .env the class is skipped with a
"not run" message. TestClient gains public EMAIL and PASSWORD constants and
isApiKeyConfigured().

Refs: DX-31579

Co-authored-by: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ef6P4M5YRs6tDbdFqkvmZc

Author: claude

src/test/java/com/contentstack/cms/TestClient.java

@@ -39,6 +39,12 @@ public class TestClient {
     public static final String ENTRY_VARIANT_LOCALE =
             isBlank(env.get("entryVariantLocale")) ? "en-us" : env.get("entryVariantLocale").trim();
 
+    /** Login email for tests that log in with a real account (null when {@code EMAIL} is not set in {@code .env}). */
+    public static final String EMAIL = env.get("EMAIL");
+
+    /** Login password for tests that log in with a real account (null when {@code PASSWORD} is not set in {@code .env}). */
+    public static final String PASSWORD = env.get("PASSWORD");
+
     /** {@code true} if {@code value} is null, empty, or whitespace-only. */
     private static boolean isBlank(String value) {
         return value == null || value.trim().isEmpty();
@@ -51,6 +57,11 @@ public static boolean isUsingDefaultStackCredentials() {
         return env.get("apiKey") == null || env.get("managementToken") == null;
     }
 
+    /** {@code true} when {@code apiKey} is set to a non-blank value in {@code .env}. */
+    public static boolean isApiKeyConfigured() {
+        return !isBlank(env.get("apiKey"));
+    }
+
     private static Contentstack instance;
     private static Stack stackInstance;