fix: shut down executeAsync executor

[arnabnandy7]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #2122 ☕️

Description

Shuts down the executor created by the zero-argument HttpRequest.executeAsync() after submitting the request. Previously, every invocation created a fixed thread pool that was never shut down, leaving its worker thread alive indefinitely.

The executor is shut down in a finally block, allowing the submitted request to finish while ensuring its worker thread terminates afterward.

Testing

• Added a regression test that executes multiple asynchronous requests and verifies that the executor threads terminate.
• Ran HttpRequestTest: 44 tests passed.
• Applied Google Java Format.
• Verified the changes with git diff --check.

No sample code was added.