The no_extension_access_modifier rule was incorrectly flagging nonisolated extension declarations as violations. The nonisolated keyword is an isolation modifier introduced in Swift 6.1, not an access control modifier, so it should not trigger this rule.

Before:

@MainActor
struct Test {}

nonisolated extension Test {} // ❌ No Extension Access Modifier Violation

After:

@MainActor
struct Test {}

nonisolated extension Test {} // ✅ No violation

Changes:

• Updated the rule logic to only check for actual access control modifiers (private, public, open, internal, fileprivate) using the existing accessLevelModifier property
• Added nonisolated extension String {} to the non-triggering examples
• Updated CHANGELOG.md with the fix

The fix leverages the existing DeclModifierSyntax.asAccessLevelModifier utility which correctly distinguishes between access control modifiers and other modifiers like isolation keywords.

Fixes #6168.

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