Reconnect 13.4-1 as an upgrade node for the 14.x path

[ihalatci]

Problem

A GA''d 13.4 cluster cannot ALTER EXTENSION citus UPDATE into the 14.x line. 13.4-1 has no incident SQL scripts, so it is not a node in the extension update graph and Postgres path-finding fails with "no update path".

multi_extension upgrade tests do not catch this because they only walk edges that are present in the graph.

Fix

Reconnect 13.4-1 as a node by adding two empty version-bump scripts, byte-identical to the pattern already shipped on release-13.2:

• src/backend/distributed/sql/citus--13.3-1--13.4-1.sql (forward)
• src/backend/distributed/sql/downgrades/citus--13.4-1--13.3-1.sql (downgrade)

Plus a no-op multi_extension test excursion (13.3 -> 13.4 -> 13.3) asserting the reconnection is a pure no-op.

The Makefile auto-globs sql/*.sql and sql/downgrades/*.sql, so the new scripts install with no Makefile change.

Verification

• Both SQL scripts are blob-hash identical to origin/release-13.2.
• multi_extension regression: clean pass, no diffs.
• The excursion leaves prev_objects at 13.3, so all downstream .out output is provably unaffected -> (0 rows).

Scope: 4 files, +19 lines -- only the 13.4-1 reconnection; no other edits.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.62%. Comparing base (152fdf4) to head (6492c68).
⚠️ Report is 3 commits behind head on release-14.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-14.0    #8650      +/-   ##
================================================
- Coverage         88.63%   88.62%   -0.02%     
================================================
  Files               288      288              
  Lines             64115    64120       +5     
  Branches           8045     8045              
================================================
- Hits              56831    56826       -5     
- Misses             4965     4971       +6     
- Partials           2319     2323       +4     

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