[exporter/prometheusremotewrite] 5xx errors incorrectly classified as error_permanent="true"

[bogdan-st]

Description

execute() had an unconditional return consumererror.NewPermanent(err) that overrode the error classification already set by executeFunc, which correctly returns plain errors for 5xx and permanent errors for 4xx. On top of that, handleRequests() wrapped every error from execute() in another consumererror.NewPermanent, unlike handleRequestsV2() which propagates errors as-is. Both cause 5xx responses to appear as error_permanent="true" on otelcol_exporter_send_failed_metric_points_total, making the attribute useless for distinguishing backend failures from data errors.

Link to tracking issue

Fixes #48431

Testing

Added two cases to TestRetries covering execute() directly: 5xx with retry disabled returns a non-permanent error, 4xx still returns permanent.