JIT: preserve receiver null checks in tail-recursion optimization

[Copilot]

Tail-recursion-to-loop conversion drops GTF_CALL_NULLCHECK on non-virtual callvirt calls, allowing null receivers to silently succeed.

• JIT: Require !call->NeedsNullCheck() for loop conversion. Otherwise, retain the fast-tail-call path, which emits the receiver null check.
• Regression coverage: Exercise null and non-null receivers at zero, one, and multiple recursion depths. Disable caller optimization to prevent inlining from masking the bug without suppressing the recursive method’s tail call.

• Fixes JIT: (bug) Tail-recursion-to-loop conversion drops the callvirt receiver null check #133861

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[EgorBo]

This fix looks too conservative, probably needs human PR