Fix #4575: project mt_created_at onto MapTo'd member in v9

[jeremydmiller]

Closes #4575.

Bug

In Marten 9.x, m.CreatedAt.MapTo(d => d.CreatedOn) configures the column but the mapped property stays at default(DateTimeOffset) after a load:

options.Schema.For<WebToSmsProgress>()
    .Metadata(m =>
    {
        m.LastModified.MapTo(f => f.LastModifiedOn);   // works
        m.CreatedAt.MapTo(f => f.CreatedOn);            // broken in v9
        m.Revision.MapTo(f => f.Version);               // works
    });

Worked in v8, regressed in v9.

Cause

The closed-shape storage rewrite (#4498 — "Scrub all remaining JasperFx.CodeGeneration usages from Marten") ported every metadata column from the codegen path into DocumentStorageDescriptorBuilder except CreatedAt. The column still gets Enabled = true and is selected from the DB (CreatedAtColumn.ShouldSelect returns Member != null), but no binder copies the value from the reader to the mapped member after deserialization.

Confirmed via diff inspection: DocumentStorageDescriptorBuilder wires TenantId, LastModified, CorrelationId, CausationId, LastModifiedBy, Headers, duplicated fields, and soft-delete; CreatedAt is the only ISelectableColumn metadata column with no binder.

Fix

Add DocumentCreatedAtBinder<TDoc> (mirroring DocumentLastModifiedBinder / DocumentTenantIdBinder) and add it to readBinders right after the LastModified slot — mt_created_at sits between mt_last_modified and mt_dotnet_type in the table (DocumentTable.cs:51); mt_dotnet_type is not ISelectableColumn, so in the SELECT projection the column lands immediately after mt_last_modified.

The binder is read-only, deliberately not added to writeBinders:

• CreatedAtColumn carries a transaction_timestamp() DEFAULT, so PostgreSQL fills the value on insert.
• CreatedAt is immutable thereafter; participating in writeBinders would put the column into the UPDATE SET list and clobber the original creation time on every subsequent save.

Test

when_mapping_to_the_created_at_metadata added to flexible_document_metadata.cs — uses the existing FlexibleDocumentMetadataContext fixture. Two new cases:

• created_at_is_populated_on_query_only — the primary regression: confirms the mapped member is non-default after reload.
• created_at_is_immutable_across_updates — pins the read-only invariant so a future change adding CreatedAt to writeBinders would fail loudly.

Both pass locally (10/10 in the new fixture, including the 8 inherited from the base context).

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