Fix union overload signature ordering instability

[ADİL ALPEREN ÇİFTCİ (adilalperenciftci)]

Fixes #53614

Expands signatures with optional parameters into exact-arity variants during getUnionSignatures so candidate overloads resolve naturally in findMatchingSignatures without ad-hoc post-sorting.

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[ADİL ALPEREN ÇİFTCİ (adilalperenciftci)]

@microsoft-github-policy-service agree

[Jake Bailey (jakebailey)]

This seems a bit too ad-hoc and out of place for the kind of thing we normally do. It is suspicious that nothing else changed, either, and that it's purely additive?

[ADİL ALPEREN ÇİFTCİ (adilalperenciftci)]

Good point on the post-sorting. I dropped that approach and replaced it with a simpler change in getUnionSignatures: signatures with optional params now expand into their exact-arity variants when union signatures are built, so findMatchingSignatures matches naturally without extra sorting passes.