fix: always include expiration field in service-account create request

[Copilot]

admin service-account create fails when --expiry is omitted because skip_serializing_if = "Option::is_none" on the expiry field drops the expiration key entirely from the JSON body. RustFS requires the field to be present.

• Remove skip_serializing_if from CreateServiceAccountRequest.expiry so it serializes as null when unset
• Add unit tests for both None and Some serialization paths

Before:

{"accessKey":"abc","secretKey":"abc"}

After:

{"accessKey":"abc","secretKey":"abc","expiration":null}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>admin service-account create returns Error specifying missing expiration</issue_title>
<issue_description>```
rc.exe admin service-account create abc abc
✗ Failed to create service account: Invalid path: InvalidRequestunmarshal body failed, e: Error("missing field expiration", line: 1, column: 37)

CLI : v0.1.4
RustFS : 1.0.0-alpha.85</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes admin service-account create returns Error specifying missing expiration #22

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[Copilot]

Pull request overview

This PR fixes admin service-account create requests against RustFS by ensuring the JSON body always includes the expiration field, even when --expiry is omitted, and adds tests to prevent regressions.

Changes:

• Remove skip_serializing_if = "Option::is_none" from CreateServiceAccountRequest.expiry so it serializes as "expiration": null when unset.
• Add unit tests covering both expiry: None and expiry: Some(...) serialization paths.

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