feat: improved async performance with thread-safe type

[petruki]

This pull request refactors how history execution is managed in the SwitcherBuilder and its concrete implementation SwitcherRequest, improves thread safety, and simplifies null checks across the codebase. The main theme is to make the history execution logic more robust and concurrent, while also standardizing null checks using Objects.nonNull and Objects.isNull.

History Execution Refactor

• Moved the historyExecution map from the abstract SwitcherBuilder to the concrete SwitcherRequest class, and changed its implementation from a regular HashMap to a thread-safe ConcurrentHashMap for better concurrency support. ([[1]](https://github.com/switcherapi/switcher-client-java/pull/381/files#diff-ac0544cec82f9e6e850d4929bf6edf6ba3397237b1bba0012f503cba484971e7L19-L20), [[2]](https://github.com/switcherapi/switcher-client-java/pull/381/files#diff-50b643c046fcf01d65f95fbeb497f0da0daaf755828ef185e1a3bd8381b2b710R27-R28), [[3]](https://github.com/switcherapi/switcher-client-java/pull/381/files#diff-50b643c046fcf01d65f95fbeb497f0da0daaf755828ef185e1a3bd8381b2b710R48))
• Changed the flush method in SwitcherBuilder to be abstract, with SwitcherRequest now implementing it to clear both entry and historyExecution. ([[1]](https://github.com/switcherapi/switcher-client-java/pull/381/files#diff-ac0544cec82f9e6e850d4929bf6edf6ba3397237b1bba0012f503cba484971e7L45-R42), [[2]](https://github.com/switcherapi/switcher-client-java/pull/381/files#diff-50b643c046fcf01d65f95fbeb497f0da0daaf755828ef185e1a3bd8381b2b710R72-R78))

Thread Safety and Synchronization

• Removed the synchronized keyword from the execute method in AsyncSwitcher, potentially improving performance and relying on the thread safety of the underlying data structures. ([src/main/java/com/switcherapi/client/model/AsyncSwitcher.javaL49-R49](https://github.com/switcherapi/switcher-client-java/pull/381/files#diff-c4e53fc6b375fc5ce77e080aa254d0321fef50ebdb25a7ccf460bbf9a2c4041dL49-R49))

[Before] Operations /s in 5s

Benchmark	Type	Mode	Score	Units
ClientJavaBenchmark.testSwitcherRemoteThrottle	Async	thrpt	40,171,215.842	ops/s
ClientJavaBenchmark.testSwitcherLocal	Local	thrpt	19,689,072.262	ops/s

[After] Operations /s in 5s

Benchmark	Type	Mode	Score	Units
ClientJavaBenchmark.testSwitcherRemoteThrottle	Async	thrpt	109,325,445.942	ops/s
ClientJavaBenchmark.testSwitcherLocal	Local	thrpt	41,501,007.578	ops/s

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.1% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud