Fix Python >= 3.13 terminal execution requiring manual Enter keypresses

[Copilot]

Problem

In Python >= 3.13, when using "Run selection/line" to execute multi-line code blocks in the terminal, users had to manually press Enter to complete execution. This was causing the REPL to show continuation prompts ("...") instead of executing the code automatically.

For example, executing this code:

def hello():
    print("Hello, World!")

Would result in:

>>> def hello():
...     print("Hello, World!")
... 

Requiring manual Enter keypresses to execute, instead of running automatically.

Root Cause

The issue was in the bracket paste mode logic for Python >= 3.13:

1. normalizeSelection.py correctly adds trailing newlines (\n\n) for multi-line code blocks to ensure proper execution
2. The bracket paste mode handling in helper.ts was removing these essential trailing newlines
3. Without sufficient trailing newlines, Python 3.13's new REPL behavior shows continuation prompts instead of executing

Solution

• Preserved trailing newlines: Removed the logic that trimmed trailing newlines for Python >= 3.13 bracket paste mode
• Maintained existing behavior: Simple expressions still work correctly with appropriate newlines
• Added test coverage: New test ensures multi-line code blocks preserve necessary trailing newlines

The fix is minimal and surgical - it simply preserves the correct normalization that normalizeSelection.py already provides:

• Simple expressions: 1 trailing newline (sufficient for execution)
• Multi-line blocks: 2 trailing newlines (required for Python 3.13 execution)

Testing

• ✅ All existing tests pass
• ✅ New test specifically validates the fix for multiline code blocks
• ✅ No regression in existing functionality
• ✅ Python < 3.13 behavior unchanged

Fixes #24565.

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