Consult reference resolver for a call to an unresolvable identifier

[poissoncorp]

Summary

Consult the reference resolver for a call to an unresolvable identifier.

Details

Since #2266 the base of an unresolvable reference is the Reference.Unresolvable sentinel rather
than undefined. JintCallExpression only consulted a custom IReferenceResolver when
baseValue.IsNullOrUndefined(), so a call to an undefined name (base = sentinel) skipped the
resolver and hit (Environment) baseValue, throwing InvalidCastException. A read of the same name
still went through the resolver, so only calls were affected.

This extends the existing "null-propagation helper" deviation to also fire for unresolvable
references (referenceRecord.IsUnresolvableReference), restoring the behavior the #1041 fix
established. A call to an undefined name is now routed through the resolver (e.g. resolved via
TryGetCallable) instead of throwing a CLR cast exception.

Linked issue

Fixes #2749

Test plan

• Added a unit test in Jint.Tests (Runtime/NullPropagation.CanCallUnresolvableReference)
• Ran dotnet test --configuration Release locally. NullPropagation passes on net10.0 and net472; the new test fails without the fix with the exact InvalidCastException
• ECMAScript spec change. N/A
• Interop change. N/A
• Perf change. N/A

Breaking change?

No. It restores the pre-#2266 behavior for custom IReferenceResolver implementations and does not change the public API.

[lahma]

Thank you!