Enable default oxlint correctness rules

[kmcginnes]

After migrating to oxlint in #1746, the correctness category is set to "off" with rules enabled explicitly. There are 7 default correctness rules that have violations and could not be enabled without code changes.

Goal

Fix the 39 lint violations and flip "correctness": "off" to "correctness": "error" in .oxlintrc.json, then remove the individual rule entries that are redundant with the category default.

Rules to enable (39 violations across ~10 files)

Rule	Violations	Description
no-unused-expressions	13	Ternaries and && used as statements instead of if
no-useless-fallback-in-spread	13	Unnecessary || {} in spread expressions
no-misused-spread	6	Spreading class instances or iterables in objects
no-meaningless-void-operator	4	void on already-void-returning calls
no-useless-length-check	2	Redundant .length > 0 before .some()
no-useless-default-assignment	1	Default value on non-nullish property
prefer-string-starts-ends-with	1	Use String#endsWith instead of substring comparison

Approach

Most fixes are mechanical:

• cond ? doA() : doB() → if (cond) { doA() } else { doB() }
• x && doSomething() → if (x) { doSomething() }
• ...(value || {}) → ...value (spreading falsy is safe)
• void fn() → fn() (when fn already returns void)
• .length > 0 && .some(...) → .some(...)

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