Fix signature help crash on parenthesized call expressions

[Copilot]

Fixed a nil pointer dereference crash that occurred when requesting signature help on parenthesized call expressions with specific trigger parameters.

Problem

The language server would crash with a nil pointer dereference when:

1. A user had a call expression like someCall()
2. They wrapped it in parentheses: (someCall())
3. Signature help was triggered at specific positions with triggerCharacter: "(" and triggerKind: 2

The crash occurred in GetSignatureHelpItems at line 103 when trying to access candidateInfo.candidateInfo without first checking if candidateInfo was nil.

Root Cause

The getCandidateOrTypeInfo function can return nil when it doesn't match any of the three invocation types (callInvocation, typeArgsInvocation, contextualInvocation). However, the calling code assumed candidateInfo would never be nil and attempted to access its fields directly, causing a panic.

Solution

Added a simple nil check for candidateInfo before accessing its fields:

if candidateInfo == nil {
    // We didn't have any sig help items produced by the TS compiler.
    return nil
}

This defensive programming approach prevents the crash while maintaining existing behavior for valid cases. When candidateInfo is nil, the function now returns nil (no signature help) instead of crashing.

Testing

• Added a test case signatureHelpParenthesizedCall that reproduces the crash scenario
• Verified the test fails with a panic before the fix and passes after
• Confirmed all existing signature help tests continue to pass
• Build and formatting checks pass

Fixes #1415.

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