test: harden transient runner probe recovery

[joecklau]

Summary

• make the transient runner-probe recovery test return the healthy process sentinel after its intentionally injected first failure
• remove scheduler-dependent competition between the readiness goroutine and subsequent invalid fake probes
• preserve production readiness validation and lifecycle behavior unchanged

Validation

• go test ./internal/pool -run '^TestProvisionOneRecoversFromTransientRunnerProbeFailure$' -count=1000
• go test -race ./internal/pool -run '^TestProvisionOneRecoversFromTransientRunnerProbeFailure$' -count=100
• go test ./...
• independent diff and lifecycle-safety verification