Refactor A2A performance driver structure

[MisterVVP]

Motivation

• Make the in-process C++ performance driver easier to maintain by centralizing constants, scenario names, and common types.
• Ensure measured latency does not include driver bookkeeping or contention from result collection.
• Improve concurrency primitives and reuse SDK helpers instead of duplicating logic.

Description

• Added tests/performance/a2a_performance_driver.h to centralize constants, scenario names/order, Options/ScenarioResult types, and function declarations.
• Refactored tests/performance/a2a_performance_driver.cpp to use the new header, replaced string literals with named constexpr values, and moved shared helpers into the a2a::tests::performance namespace.
• Replaced the push-delivery mutex and integer with a relaxed std::atomic<int> counter to avoid unnecessary locking on hot paths.
• Replaced std::async + shared-result mutex with a fixed worker pool that uses std::atomic<int> work distribution and per-thread result buffers so aggregation happens after measured operations finish, avoiding measurement bias from collection locks.
• Added tests/performance/README.md documenting the driver, measured fields, store backends, and example invocations.