Fix threading.Thread target misuse in Alpaca paper trading (Issue #1399)

[skyliquid22]

This PR fixes incorrect usage of threading.Thread where the result of
submitOrder(...) was passed as the thread target, causing immediate execution
instead of deferred execution inside the thread.

Changes:

• Corrected thread initialization to use target=self.submitOrder
• Passed arguments via args=(...)
• Applied fix to both buy and sell order blocks
• Added an AST-based regression test to ensure submitOrder is not called during thread construction and args are provided when used as a target.

Closes #1399