fix: preserve total_byte_size in calculate_total_byte_size when num_r…

[bert-beyondloops]

Which issue does this PR close?

• Closes Statistics::calculate_total_byte_size discards a known total_byte_size when num_rows is unknown #24026

Rationale for this change

Statistics::calculate_total_byte_size is meant to derive total_byte_size from num_rows and the schema's fixed-width columns. When all columns have a primitive width but num_rows is Precision::Absent, the old code computed
self.num_rows.multiply(&Precision::Exact(size)), and Precision::multiply returns Precision::Absent whenever either operand is Absent. This silently overwrote any previously known total_byte_size (exact or inexact) with
Absent, even though the non-primitive-width branch already handled this situation correctly by downgrading the existing value to inexact instead of discarding it.

What changes are included in this PR?

• In Statistics::calculate_total_byte_size, when the schema is all fixed-width but num_rows is Precision::Absent, keep the existing total_byte_size and downgrade it to inexact via to_inexact(), instead of overwriting it with
  Absent.
• Updated the doc comment on calculate_total_byte_size to describe this behavior.
• Added test_calculate_total_byte_size covering: an all-primitive schema with known row count (exact size), an all-primitive schema with unknown row count (preserved but downgraded to inexact), and a non-primitive schema
  (always downgraded to inexact regardless of row count).

Are these changes tested?

Yes — added `stats::tests::test_calculate_total_byte_size, exercising all three branches of the updated match.

Are there any user-facing changes?

No public API changes. Statistics propagation is more accurate (previously known total_byte_size estimates are no longer dropped to Absent when num_rows is unknown), which may result in slightly better cost-based planning
decisions in some cases.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.85%. Comparing base (dbcb5c0) to head (5496845).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24027      +/-   ##
==========================================
- Coverage   80.85%   80.85%   -0.01%     
==========================================
  Files        1101     1101              
  Lines      374933   374951      +18     
  Branches   374933   374951      +18     
==========================================
+ Hits       303166   303177      +11     
- Misses      53671    53675       +4     
- Partials    18096    18099       +3     

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[pepijnve]

This change makes sense to me since it avoids downgrading Exact total_byte_size to absent when num_rows is Absent.

[asolimando]

Intuitively this makes sense to me too, hopefully we will soon be able to have a more data-driven approach thanks to #23975.

[asolimando]

LGTM