Fix union overload signature ordering instability

[Adil Alperen Çiftci (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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[Adil Alperen Çiftci (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?

[Adil Alperen Çiftci (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.

[Adil Alperen Çiftci (adilalperenciftci)]

Rebased cleanly on current main. Whenever you get a chance, let me know if this revised getUnionSignatures arity expansion looks better to you or if you'd prefer handling the union overload matching differently.

[Adil Alperen Çiftci (adilalperenciftci)]

Hi Jake Bailey (@jakebailey), just following up on this PR. The branch is rebased cleanly on current main with no conflicts. Please let me know if the revised getUnionSignatures approach looks good to you when you have a moment to review.

[Jake Bailey (jakebailey)]

I don't think this PR is right but can't put my finger on why. Figuring out why it feels wrong probably means I have to figure out a fix myself, which defeats the purpose a bit.

[Adil Alperen Çiftci (adilalperenciftci)]

I removed the exact-arity signature expansion. Besides changing the signature set, it could panic on a tuple-rest signature when the effective parameter count exceeded the raw parameter slice length.

The replacement keeps the existing union-signature matching and derives precedence from each constituent's existing reorderCandidates order. This preserves literal specialization and merged-interface behavior without generating artificial arity variants. The regression fixture covers optional/rest parameters, tuple-rest, generics, constructors, three constituents, and merged declarations.

Validation: npm test completed with 163,505 test records, 2,152 skipped, and no failures; npm run build and npm run check:format also passed.