fix(no-unnecessary-condition): don't flag required ?. on deferred conditional return types

[pelly-ryu]

Generated with Claude Code, reviewed and tested by me.

A generic method returning a deferred conditional makes the chain below report a false "Unnecessary optional chain on a non-nullish value", even though b.get('a') is Box | null (tsc reports TS2531 without the ?.).

interface Box {
  get<H = never>(name: string): [H] extends [never] ? Box | null : Box;
}
declare const b: Box;
const r = b.get('a')?.get('b')?.get('c');

On a chain of 3+ links TypeScript leaves the inner conditional unresolved (TypeFlagsConditional), which carries no nullish flag, so isNullishType reads it as non-nullish. In isCallExpressionNullableOriginFromCallee that makes the callee return look non-nullable, the chain's | undefined is attributed to the optional-chain short-circuit, and removeNullishFromType drops the real | null. The same unresolved conditional also reaches the final nullish check in checkOptionalChain directly when the chain is property access whose type stays unresolved (a type parameter in scope).

A new isConstrainedNullishType replaces a conditional with its base constraint (the union of its branches) before the nullish test, recurses into union members, and is gated on TypeFlagsConditional so other types are unchanged. It is used at both nullishness checks the optional-chain path relies on: the callee-return check and the final check in checkOptionalChain. A deferred conditional whose branches are all non-nullish is still reported.

Adds 6 valid regression cases (| null/| undefined branch, explicit <never>, named alias, union-member conditional, property access in a generic scope) and an invalid precision case; the snapshot diff is additive.