fix(exts): handle pg_upgrade generated update_extensions.sql script in the extension tests

[jfroche]

pg_upgrade may generate an update_extensions.sql script to update extensions after a major version upgrade.
This PR modifies the extension tests to check for the presence of this script after upgrading PostgreSQL to version 17.
If the script exists, it is executed to ensure that the extensions are updated correctly.

Summary by CodeRabbit

• Tests

  • Improved PostgreSQL extension upgrade tests to detect and conditionally run extension update scripts during major-version migrations.
  • Expanded test coverage by adding additional extensions to the upgrade matrix.

• Documentation

  • Added clarifying comments in test configurations explaining why certain extensions need adapted checks or special dependency handling.

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[coderabbitai]

Walkthrough

Adds runtime detection of an update_extensions.sql script during the PostgreSQL 17 upgrade flow, conditionally runs it and adjusts extension-version assertions; inserts pg_partman and postgis into the test manifest; and adds explanatory comments for several extension tests.

Changes

Cohort / File(s)	Summary
PostgreSQL 17 upgrade & extension manifest nix/ext/tests/default.nix	Adds has_update_script check for /var/lib/postgresql/update_extensions.sql after switching to the 17 config; conditionally runs the update script and asserts extension versions from either versions["17"][-1] or the last_version recorded from 15; inserts pg_partman and postgis into the extension test manifest.
Extension-specific test notes nix/ext/tests/http.nix, nix/ext/tests/pg_repack.nix, nix/ext/tests/pgrouting.nix	Adds non-functional comments explaining why the generic test is not used for these extensions: http (non-semver versions), pg_repack (no upgrade SQL/update path), pgrouting (requires PostGIS).

Sequence Diagram(s)

sequenceDiagram
  participant TestHarness as Test Harness
  participant FS as Filesystem
  participant PG as PostgreSQL 17

  TestHarness->>PG: switch to PG 17 configuration
  TestHarness->>FS: check /var/lib/postgresql/update_extensions.sql (has_update_script?)
  alt update script present
    TestHarness->>PG: run update_extensions.sql
    PG-->>TestHarness: extensions updated
    TestHarness->>TestHarness: assert versions == versions["17"][-1]
  else no update script
    TestHarness->>TestHarness: assert versions == last_version_from_15
  end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• samrose
• LGUG2Z

Poem

🐰 I sniffed a script by moonlight bright,

Hopped between versions in the night,
If found, I nudge the upgrade through,
If not, I keep the_old_version true,
A carrot of tests, tidy and light.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main change: handling pg_upgrade's update_extensions.sql script in extension tests.
Description check	✅ Passed	The description clearly explains the purpose and scope of the changes, though it does not follow the template structure with explicit sections.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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