Fix concurrent distribution relation lock assertion

[ihalatci]

Summary

EnsureColocateWithTableIsValid called BuildDistributionKeyFromColumnName with NoLock during concurrent-distribution preflight. On cassert builds, PostgreSQL's try_relation_open asserts that callers using NoLock already hold at least AccessShareLock, but this path held no relation lock.

Acquire AccessShareLock through the existing distribution-key helper and release it immediately after extraction. This keeps preflight lock retention short and preserves the later ShareUpdateExclusiveLock acquisition and distribution-column revalidation.

Validation

• PG16.14 --enable-cassert: focused create_distributed_table_concurrently regression passed.
• PG17.10 --enable-cassert: focused create_distributed_table_concurrently regression passed.
• The exact colocate_with := 'nocolo' case returns the expected distribution-column type mismatch with no assertion or signal 6.

The focused runs used check-base EXTRA_TESTS=create_distributed_table_concurrently; the target passed on both versions. The overall harness command remained nonzero because three unrelated out-of-tree fixture tests (multi_cluster_management, multi_create_table_superuser, and tablespace) failed due to the archived source layout.

Fixes #8671
Related to #8665

Fixes: #8672

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.65%. Comparing base (f91592f) to head (4c6ef3f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8679      +/-   ##
==========================================
- Coverage   88.73%   88.65%   -0.08%     
==========================================
  Files         288      288              
  Lines       64428    64429       +1     
  Branches     8113     8114       +1     
==========================================
- Hits        57167    57117      -50     
- Misses       4917     4971      +54     
+ Partials     2344     2341       -3     

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