#27862 fix(cmux-next): VNC reconnect test helpers are nonisolated (CmuxNextAppTests compile red)

#26557 landed RemoteVNCReconnectTests (carried from #26697). Under the CmuxNext test target's .defaultIsolation(MainActor.self), the private AttemptCounter is main-actor isolated, so the @Sendable transport factory cannot call increment():

RemoteViewVNCTests.swift:130:22: error: call to main actor-isolated instance method 'increment()' in a synchronous nonisolated context

CmuxNextAppTests does not compile on feat-cmux-next at 0ce3c31ee53, so every CmuxNext suite run fails at build. This marks the two lock-guarded test helpers nonisolated, as #27811 already writes them (that PR now conflicts with base). Two lines, tests only.

Observed / Not run

• Fleet focused run at feat-cmux-next 57b38a2-based head: test target fails to compile with the error above.
• Fleet focused run at this head (RemoteVNCReconnectTests, RemoteVNCMappingTests, PaneTabBarControlsTests, NewTabMenuTriggerTests): result posted in a comment below.

Changelog

none

Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Fixes the CmuxNextAppTests compile failure on feat-cmux-next by marking two VNC reconnect test helpers as nonisolated.

Under the test target's MainActor default isolation, the @Sendable transport factory could not call AttemptCounter.increment(), breaking the build for every CmuxNext suite run. The lock-guarded CountingRFBTransport and AttemptCounter helpers are now nonisolated; test behavior is unchanged.