pg19 - Restore tdigest aggregate test plan intent

[ihalatci]

Summary

• disable citus.allow_aggregate_worker_combine_on_internal_types in tdigest_aggregate_support so the regression test continues to assert tdigest-specific distributed plan shapes instead of the generic internal-state worker-combine path
• update the primary expected output and the existing _0/_1 alternatives with the new SET echo
• keep the existing alternative semantics: _0 covers extension-absent output and _1 covers accepted tdigest result-value variation
• account for PostgreSQL 19 Beta 2's undefined-function DETAIL through the normalization already present on the refreshed pg19-support base (ca86a5d95); a separate _2 alternative was validated during development but removed because that base normalization makes it redundant and unreachable

Validation

• -Werror builds: PostgreSQL 17.10, 18.4, and 19 Beta 2
• focused tdigest_aggregate_support schedule with tdigest installed: passed on PostgreSQL 17.10, 18.4, and 19 Beta 2
• focused schedule without tdigest: PostgreSQL 17.10 and 18.4 matched _0; PostgreSQL 19 Beta 2's raw DETAIL normalized and matched _0
• make check-style
• git diff --check

Closes #8667

This PR intentionally targets the non-default pg19-support branch. GitHub may not automatically close #8667 when this merges there; closure should be confirmed when the support branch is subsequently integrated into the default branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.80%. Comparing base (cfb4063) to head (9869797).

Additional details and impacted files

@@              Coverage Diff              @@
##           pg19-support    #8681   +/-   ##
=============================================
  Coverage         88.79%   88.80%           
=============================================
  Files               288      288           
  Lines             64416    64416           
  Branches           8101     8102    +1     
=============================================
+ Hits              57201    57205    +4     
+ Misses             4877     4874    -3     
+ Partials           2338     2337    -1     

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