feat(phase4): add comprehensive async tool calling tests

Phase 4: Testing and Validation - COMPLETED ✅

Added Tests (15 total, all passing):
=================================
1. AsyncToolCallbackTest.java (8 tests)
   ✅ testCallAsyncReturnsExpectedResult
   ✅ testCallAsyncWithDelay
   ✅ testSupportsAsyncDefaultIsTrue
   ✅ testSupportsAsyncCanBeOverridden
   ✅ testSynchronousFallbackCallBlocksOnAsync
   ✅ testSynchronousFallbackWithDelayedAsync
   ✅ testAsyncErrorHandling
   ✅ testAsyncCallbackWithReturnDirect

2. DefaultToolCallingManagerAsyncTests.java (7 tests)
   ✅ testExecuteToolCallsAsyncWithAsyncToolCallback
   ✅ testExecuteToolCallsAsyncWithSyncToolCallback
   ✅ testExecuteToolCallsAsyncWithMixedTools
   ✅ testExecuteToolCallsAsyncWithReturnDirectTrue
   ✅ testExecuteToolCallsAsyncWithMultipleToolsReturnDirectLogic
   ✅ testExecuteToolCallsAsyncWithAsyncToolError
   ✅ testExecuteToolCallsAsyncWithNullArguments

Interface Updates:
==================
- Added call(String) default method to AsyncToolCallback
- Ensures backward compatibility with ToolCallback interface
- Proper delegation chain: call(String) -> call(String, ToolContext) -> callAsync()

Test Coverage:
==============
✅ Async tool execution
✅ Sync tool fallback
✅ Mixed async/sync tools
✅ Error handling
✅ ReturnDirect logic
✅ Null/empty arguments handling
✅ Tool context support
✅ Delay testing

All tests passing: 15/15 (100%)

Related: #async-tool-support

Author: JGoP-L