test: add integration test for PostgreSQL declarative partitioned tables

[jsuhani-2026]

Added a new end-to-end integration test (PostgreSQLPartitionedTablesIT) to verify that the SourceDbToSpanner template correctly migrates partitioned tables from a PostgreSQL source into Cloud Spanner.

It validates data ingestion for all three PostgreSQL partitioning strategies across both GoogleSQL and PostgreSQL Spanner dialects:

• RANGE partitioning (measurements_range table)
• LIST partitioning (employees_list table)
• HASH partitioning (orders_hash table)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.49%. Comparing base (06d6601) to head (9fed72d).

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #4016       +/-   ##
=============================================
+ Coverage     35.58%   63.49%   +27.91%     
- Complexity      697     2508     +1811     
=============================================
  Files           250      535      +285     
  Lines         17049    30720    +13671     
  Branches       1729     3420     +1691     
=============================================
+ Hits           6067    19506    +13439     
+ Misses        10473    10202      -271     
- Partials        509     1012      +503     

Components	Coverage Δ
spanner-templates	88.08% <ø> (∅)
spanner-import-export	∅ <ø> (∅)
spanner-live-forward-migration	89.23% <ø> (∅)
spanner-live-reverse-replication	83.41% <ø> (∅)
spanner-bulk-migration	92.44% <ø> (∅)
gcs-spanner-dv	90.00% <ø> (∅)
see 419 files with indirect coverage changes

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