Context

We need to add the h field alongside the existing sz field in the m size-metadata oplog sub-document created from the Replicated Size and Count project (SPM-4163).
This ticket only adds the field to the oplog entry. Nothing should produce or read h yet.

m (cpp name sizeMetadata) is an optional oplog-entry field whose value is a variant: a SingleOpSizeMetadata for a single insert/update/delete, or an array<MultiOpSizeMetadata> for commitTransaction.

Add h to SingleOpSizeMetadata only so it lands on i/u/d entries (and transaction inner ops, which use the same struct).

h must be optional. This is because m can legitimately carry sz without h (older binaries, feature-off, FCV downgrade, unsupported collections), and a required field would fail to parse those entries.

Acceptance criteria

• h added to SingleOpSizeMetadata as optional: true, as a int32 safeInt.
• Round-trip test: a SingleOpSizeMetadata with only __ sz (no h) parses without error (this may exist).
• Round-trip test: one with both sz and h serializes and parses back equal.
• No producer or consumer wired
• Marked downstream-impacting; tools team notified per the Downstream Impact process.