Vector3: Fix set z optionality and reflect parameter name

[eunjiyun]

Summary

Fix declarations for Vector3.set() and Vector3.reflect().

Evidence

JS implementation:

• three.js/src/math/Vector3.js:84 (set)
• three.js/src/math/Vector3.js:922 (reflect)

Declaration:

• types/three/src/math/Vector3.d.ts:53 (set)
• types/three/src/math/Vector3.d.ts:219 (reflect)

Observed mismatch:

• set(): z has a default fallback in JS but is declared as required in the type declaration.
• reflect(): Parameter name does not match the JS implementation and JSDoc.

Local Check

pnpm test  # failed: unrelated ESLint plugin error

Scope

• Declaration file only
• No runtime change
• Both fixes are isolated to Vector3.d.ts