Fix PG19 CDC catalog access

[ibrahim halatci (ihalatci)]

Summary

• mark only pg_dist_partition, pg_dist_shard, and pg_dist_local_group as user_catalog_table=true for PostgreSQL 19 logical decoding
• preserve those reloptions across downgrade so Citus CDC remains usable while PostgreSQL stays on 19
• document the paired downgrade as an intentional no-op; no unrelated metadata is marked

Validation

• PostgreSQL 19 focused CDC 001/002/003/005/006: 16 tests passed
• PostgreSQL 19 14.x -> 15 -> 14.x cycle retained all three reloptions; CDC insert/update/delete/truncate passed after downgrade; re-upgrade and subsequent decoding passed
• PostgreSQL 17.10 focused CDC 001/002/003/005/006: 16 tests passed
• PostgreSQL 18.4 focused CDC 001/002/003/005/006: 16 tests passed
• multi_extension: passed
• migration pairing and generated-artifact audits: passed

Closes #8735

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (ff8f1f8) to head (f6aca4f).

Additional details and impacted files

@@               Coverage Diff                @@
##           pg19-support    #8740      +/-   ##
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- Coverage         88.80%   88.79%   -0.02%     
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  Files               288      288              
  Lines             64416    64416              
  Branches           8100     8102       +2     
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- Hits              57206    57199       -7     
- Misses             4873     4878       +5     
- Partials           2337     2339       +2     

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