Reduce CI test parallelism cap from 8 to 4

[rhysparry]

Summary

• Lowers LevelOfParallelismInTeamCity() cap from 8 to 4 in CustomLevelOfParallelismAttribute. On our 8 vCPU Standard_D8as_v4 agent this drops in-flight tests from 6 to 4.
• Updates the doc-comment table in the same file to reflect the new mapping.
• Goal: reduce contention-driven flakiness on CI agents — particularly the "connection forcibly closed" / TcpClientManager dispose races and Stopwatch-vs-wall-clock drift seen on Windows 2012R2 under load. Filed in isolation (off main, not stacked on EFT-3214's other branch) so the effect on the next CI run is observable on its own.

Test plan

• CI runs to completion on Windows 2012R2 net8.0 agent
• Compare failed-test count vs recent main-nightly and the previous PR Fix race condition in ObserveUnauthorizedPollingConnections test #713 build
• Spot-check a couple of timing-sensitive tests (HalibutTimeoutsAndLimits_AppliesToTcpClientReceiveTimeout, WhenThenNetworkIsPaused_*) for whether Stopwatch.Elapsed assertions now hold
• Confirm overall test wall-clock isn't substantially worse (we expect some increase from less parallelism)

🤖 Generated with Claude Code

[rhysparry]

Test run completed first run in ~28m.