C#: Add null-conditional (?.) support to CSharpPattern

[knutwannheden]

Summary

CSharpPattern could not distinguish between regular member access (.) and null-conditional member access (?.). This meant a pattern like {obj}?.ToString() would incorrectly match x.ToString(), and {obj}.ToString() would incorrectly match x?.ToString(). Recipe authors targeting null-conditional expressions had no way to be precise about which access style they wanted to match.

The root cause: PatternMatchingComparator skips all Markers during structural comparison, but ?. is represented as a NullSafe marker on the FieldAccess.Name / MethodInvocation.Name identifier — so the two forms were structurally identical to the comparator.

After this change, patterns correctly distinguish the two forms:

var obj = Capture.Of<Expression>("obj");

// Matches only x?.ToString(), not x.ToString()
var nullConditional = CSharpPattern.Create($"{obj}?.ToString()");

// Matches only x.ToString(), not x?.ToString()
var regular = CSharpPattern.Create($"{obj}.ToString()");

Changes

• PatternMatchingComparator: Check NullSafe marker presence when comparing nodes. If the pattern node has a NullSafe marker, the candidate must too (and vice versa).
• PatternMatchTests: 5 new tests covering null-conditional method calls, field access, exact matching, and negative cases for both directions (. pattern vs ?. candidate and ?. pattern vs . candidate).

Test plan

• All 67 PatternMatchTests pass (62 existing + 5 new)
• CI green