fix(compactor): skip retention scans for zero retention

[u-kai]

What this PR does / why we need it:

This fixes retention scan planning so a retention period of 0 is treated as disabled when computing
the latest retention start time.

Which issue(s) this PR fixes:
Fixes #21755

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

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Note

Medium Risk
Changes how retention cutoffs are computed when retention is set to 0, which can affect whether index tables are scanned for deletions and could unintentionally skip/perform scans if miscomputed.

Overview
Treats retention periods of 0 (or non-positive) as retention disabled when planning compactor retention scans.

findLatestRetentionStartTime now ignores non-positive stream retention values, avoids computing negative cutoffs when defaults/users have retention disabled, and records per-tenant disabled retention as 0 so IntervalMayHaveExpiredChunks won’t fall back to default retention.

Expands unit tests to cover zero-retention combinations for default, stream-level, and per-tenant retention, including ensuring disabled per-tenant retention does not trigger index scans.

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