fix(collector): keep collecting temperatures without CPU power status

[SaiPisey2]

Addresses #2906.

Apple Silicon does not implement IOPMCopyCPUPowerStatus, so fetchCPUPowerStatus gets back kIOReturnNotFound. Update returned that error straight away, which meant it never reached updateTemperatures, so no temperature metrics were collected at all. The error also isn't ErrNoData, so it was logged at error level on every scrape.

Measured on an M5 Pro (darwin/arm64) against master:

fetchCPUPowerStatus()      -> status=map[] err=no CPU power status has been recorded
Update()                   -> err=no CPU power status has been recorded ; metrics emitted=0
IsNoDataError(err)         -> false
updateTemperatures() alone -> err=<nil> ; temperature metrics available=52

52 usable temperature sensors on that machine, none of them exported, because an expected and unrelated condition aborted the collector.

Change

Treat kIOReturnNotFound as "this system does not report CPU power status" rather than a failure: skip the three CPU power metrics, log at debug level, and continue on to the temperature sensors. Any other non-success return code is still returned as an error, exactly as before, so systems that do report CPU power status are unaffected.

After the change, on the same machine, Update() returns no error and emits all 52 temperature metrics.

Scope

This does not make the CPU power metrics appear on Apple Silicon and does not resolve #2218 — the underlying API provides no data there, as @rexagod already established on #2906. It only stops their absence from suppressing the temperature metrics. I've left Closes #2906 out of the commit deliberately, since whether that issue is fully answered by this is a maintainer call.

Testing

Added collector/thermal_darwin_test.go, which fails on master:

--- FAIL: TestThermalUpdateWithoutCPUPowerStatus
    thermal_darwin_test.go:44: Update returned errNoCPUPowerStatus; a system
    without CPU power status must still collect temperatures

and passes with the change. Also verified locally:

• go build ./...
• go vet ./collector/
• gofmt -l clean on both touched files
• go test ./collector/ full package
• go build -tags notherm ./collector/