fix: accept day suffix in validityDuration as documented

[xrl]

Setting validityDuration to the documented day format (e.g. 365d, used by both TLS samples in config/samples) makes reconciliation fail permanently with failed to parse ValidityDuration: time: unknown unit "d" in duration "365d", because parseValidityDuration delegates directly to time.ParseDuration, which does not support the d unit the API doc promises (100d12h). This change expands a leading day segment to hours before parsing, leaving plain Go durations and the empty-string default untouched. Adds table-driven tests for parseValidityDuration covering day-suffix, mixed, plain, default, and invalid inputs.

[kubernetes-prow]

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This pull-request has been approved by: xrl
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[kubernetes-prow]

Hi @xrl. Thanks for your PR.

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