fix(format): preserve model column dialect - #5926

@fresioAS

Sharing some findings from digging into this area. They support the approach here and suggest a way to extend it.

1. columns isn't the only property that lost its dialect. Rendering the whole header generically affects every property whose value is user-authored SQL. On main today, a T-SQL model with audits (my_audit(threshold := CAST('2024-01-01' AS DATETIME2))) formats to '2024-01-01'::TIMESTAMP, the same silent type downgrade as columns, just unreported so far. The same applies to expression values in physical_properties, and to time_data_type nested inside the SCD kinds. So the columns-only fix leaves some corruption in place.

2. Probably don't need a hand-maintained property list. ModelMeta already declares which properties hold expressions (columns, audits, signals, partitioned_by, physical_properties, …) versus which are SQLMesh's own scalars (allow_partials, description, kind, …). Deriving the policy from those annotations classifies all of them, and the failure mode inverts usefully: a property that's missed doesn't get canonicalized, rather than having a user's SQL silently rewritten.

I prototyped this as a one-condition change to the hook in this PR. Swapping the columns name check for a policy lookup - extended to AUDIT/METRIC headers and nested kinds. It fixes the audits/physical_properties/time_data_type cases too, and sqlmesh format --transpile starts converting column types correctly again while keeping booleans safe. No regressions found with this approach.

Happy to open it as a follow-up on top of this, or hand it over if you'd rather fold it in. This PR's diagnosis and hook design are what it's built on.

One process note independent of the approach: whatever lands is worth calling out as a behavior change in the release notes. This would be the third consecutive release that reformats T-SQL headers