fix(migrations): stop migrator.up() from churning a snapshot that matches the DB

[B4nan]

Follow-up to #7803 (now merged) for the Postgres side of #7798.

migration:create writes .snapshot-<db>.json from entity metadata, while migration:up/migration:down rewrite it from DB introspection. Even when the migrated database still matches the snapshot, the two serializations diverge in cosmetic-only ways the SchemaComparator already treats as equal:

• current_timestamp → CURRENT_TIMESTAMP (default casing)
• (coalesce(...) ...) STORED → (COALESCE(...) ...) stored (generated-expr reformatting)
• partial-where clauses rewrapped with explicit casts (status = 'PENDING' → (status)::text = 'PENDING'::text)
• native-enum mappedType, index gin method, check definition format

So every migrator.up() produced a noisy git diff on the snapshot even with no real schema change (the reported symptom). These can't all be normalized byte-for-byte in toJSON() — Postgres rewrites the stored expressions unpredictably.

Instead, skip the rewrite when the introspected schema doesn't differ from the on-disk snapshot in any way the comparator turns into SQL. The file then stays exactly as authored by migration:create. A real schema change (or a stale snapshot) still rewrites it as before — the existing sqlite up/down snapshot test is updated to assert this no-churn guarantee.

Closes #7798

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.64%. Comparing base (a972992) to head (4a33621).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #7804    +/-   ##
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  Coverage   99.64%   99.64%            
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  Files         285      285            
  Lines       29664    29671     +7     
  Branches     8119     8291   +172     
========================================
+ Hits        29559    29566     +7     
  Misses         98       98            
  Partials        7        7            

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