fix(client): support graph-scoped legacy user auth paths

[looksaw2]

Summary

• use graphs/{graph}/auth/users when the server does not support GraphSpace;
• request all legacy users with limit=-1, then find the exact username in the Server 1.5 users wrapper;
• preserve the existing GraphSpace-scoped path and name query for modern servers;
• add regression coverage for the legacy path, a target user after the default first 100 records, and the unchanged modern path.

Environment and reproduction

• HugeGraph Server: 1.5.0, Core 1.5.0, REST API 0.71.0.0
• Authentication: StandardAuthenticator
• Context: DEFAULT / hugegraph
• PR base: apache/hugegraph-toolchain@be7ef3ae
• OS: macOS

The full Before flow and three step-by-step screenshots are recorded in #759.

After verification

1. Real Server 1.5 contract:
   • GET /graphs/hugegraph/auth/users?limit=-1 -> HTTP 200
   • GET /auth/users?limit=-1 -> HTTP 404
   • response wrapper -> users
2. Patched Java Client against the real Server 1.5 runtime:
   • client.auth().getUserByName("admin") -> user=admin success=true
3. Regression suite:
   • 63 tests, 0 failures, 0 errors
   • 0 Checkstyle violations

Step 1: verify the real Server 1.5 contract

Step 2: run the patched Java Client against Server 1.5

Step 3: run focused and unit regression tests

Tests

cd hugegraph-client
mvn test -Dtest=AuthApiPathTest,UnitTestSuite \
  -Dmaven.javadoc.skip=true -ntp

Fixes #759