Refactor conditional loading of ThreadPoolExecutor

[alex-pewpew]

Description of changes:

• Move ThreadPoolExecutor imports from lazy to "not as lazy". It offloads unnecessary work from actual invocations, especially the first one. Existing tests are already covering affected code paths and components
• ThreadPoolExecutor is still imported as a private dependency of a LambdaRuntimeClient, not exposing the symbols imported conditionally
• Fix tests - the latest runtimes have changed unittest.mock.patch behavior, when sys.stdout patching does not work as expected in certain combinations with other @patch'es in our testcases

Target (OCI, Managed Runtime, both): Both

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