Fix signature help crash on malformed function declarations

[Copilot]

This PR fixes a crash that occurs when signature help is triggered on malformed function declarations with trailing commas but missing closing parentheses.

Issue Description

The crash happened when users were defining a function and added a comma at the end without having a closing parenthesis:

sub test(p1 = invalid, <-- adding this comma causes the crash
  result = 1
end sub

When signature help was triggered (by typing comma or opening parenthesis), the language server would crash with a TypeError: Cannot read properties of undefined (reading 'parent').

Root Cause

The crash occurred in CallExpressionInfo.ts when the code tried to access expression.parent without checking if it was null/undefined. For malformed function declarations, the AST parser creates partial nodes where some parent relationships might not be established, leading to null reference errors.

Solution

Added comprehensive null safety checks in CallExpressionInfo.ts:

1. Fixed null reference in ascertainCallExpression(): Added optional chaining for expression.parent?.findAncestor()
2. Added safety check in isPositionBetweenParentheses(): Return false early if callExpression is null
3. Fixed null reference in process(): Added null check before accessing callExpression.parent
4. Enhanced getParameterIndex(): Added safety checks for null callExpression or args

Testing

Added three comprehensive test cases covering:

• Malformed function declaration with trailing comma (original issue)
• Incomplete function calls
• Malformed function declaration with missing closing parenthesis

All existing tests continue to pass (2441 passing), ensuring no regressions were introduced.

Verification

Manual testing confirms that signature help no longer crashes on malformed code and returns an empty array gracefully instead of throwing an exception.

Fixes #1467.

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