cloudstorage: make all DateSeparator fields use the enum instead of string (#6078)

[ti-chi-bot]

This is an automated cherry-pick of #6078

What problem does this PR solve?

Issue Number: close #6077

What is changed and how it works?

Changes all places that uses the DateSeparator expecting a string to the common enum config.DateSeparator so case-sensitivity no longer appears.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Questions

Will it cause performance regression or break compatibility?

No

Do you need to update user documentation, design documentation or monitoring documentation?

No

Release note

Previously sink.date-seperator accepted non-lowercased values like "DAY" but ignored them. Now they are accepted and treated the same as the lowercased versions.

Summary by CodeRabbit

• Improvements
  • Standardized date-separator configuration using supported typed values across storage sinks.
  • Added consistent JSON and TOML serialization, including case normalization and validation of unsupported values.
  • Improved date-separator handling when parsing storage paths and generating dated files.
  • Cloud-storage cleanup now reports configured separators more clearly.
  • Sensitive configuration masking now also covers large-message storage credentials.
• Bug Fixes
  • Invalid date-separator settings are now rejected with clear configuration errors.
  • Storage consumers consistently apply the configured separator during scans.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick:

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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