test(java): add async-specific tests for ConsumerGroupsTcpClient

[atharvalade]

Which issue does this PR close?

Closes #2225

Rationale

The async ConsumerGroupsTcpClient had no dedicated test coverage. While the blocking client had ConsumerGroupsClientBaseTest and ConsumerGroupsTcpClientTest, the async path only had a few scattered tests inside the monolithic AsyncClientIntegrationTest. Dedicated tests are needed to verify correct CompletableFuture semantics.

What changed?

The async consumer groups client had no focused test class, leaving join/leave, error propagation, and concurrent CompletableFuture patterns untested.

Added AsyncConsumerGroupsTest.java with 17 tests covering all six API operations (create, get by id, get by name, list, delete by id, delete by name), join/leave membership including multi-client scenarios, error scenarios that assert IggyResourceNotFoundException propagates correctly through ExecutionException, and CompletableFuture-specific patterns such as thenCompose chaining and allOf concurrent operations.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. Minimal usage: used for codebase exploration and initial scaffolding. All test logic, assertions, and cleanup patterns were reviewed and verified against existing test classes.
3. Ran the full test suite locally via ./gradlew :iggy:test confirming 17/17 tests pass with 0 failures and no Netty resource leaks.
4. Yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.44%. Comparing base (3b29d33) to head (999e808).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2890      +/-   ##
============================================
+ Coverage     70.39%   70.44%   +0.04%     
- Complexity      772      776       +4     
============================================
  Files          1041     1041              
  Lines         86437    86437              
  Branches      62706    62707       +1     
============================================
+ Hits          60850    60887      +37     
+ Misses        23064    23029      -35     
+ Partials       2523     2521       -2     

Flag	Coverage Δ
java	56.26% <ø> (+0.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 3 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.