Add robust fallback for goal conversion totals in CalculateConversionPageRate

[sgiehl]

Description

CalculateConversionPageRate depends on top-level goal totals from archive numerics.
In some contexts these totals can be missing/zero, causing page conversion rates to be skipped or incorrect.

If archive totals are unavailable, using reliable totals present in the table allows rate calculation to proceed safely.

refs #24191

Checklist

• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• New documentation added or existing documentation updated

[sgiehl]

Closing for now. While the solution somewhat works, it just feels incorrect. The problem is somewhere else and shouldn't be fixed that way.