Use manifest-driven SQL test ordering and manifest coverage guard

[seanwevans]

Motivation

• Replace the fragile, hand-maintained TESTS := ... list in makefile with a single authoritative source of truth so test order is deterministic and easier to update.
• Ensure contributors cannot accidentally add test/sql/*_test.sql files without registering them in the execution order.

Description

• Add test/sql/manifest.txt as the authoritative, ordered list of tests and include bootstrap entries like test/sql/init.sql.
• Change makefile to read TESTS from TEST_MANIFEST := test/sql/manifest.txt using sed and preserve ordered execution by feeding that list to pg_prove.
• Add a check-test-manifest make target that fails when any discovered test/sql/*_test.sql file is missing from the manifest, using a comm/find/sed check, and make test depend on this guard.
• Update README.md Testing section to document the manifest-driven mechanism and make test invocation, and update CONTRIBUTING.md with guidance to add new tests by adding the file and registering its path in test/sql/manifest.txt and running make check-test-manifest.

Testing

• Attempted make check-test-manifest and make -n test but the local environment lacks the PGXS include path (error: /usr/lib/postgresql/16/lib/pgxs/src/makefiles/pgxs.mk) so the Makefile-based run could not complete.
• Validated the coverage guard logic externally with the shell check comm -23 <(find test/sql -maxdepth 1 -type f -name '*_test.sql' | sort) <(sed -e '/^[[:space:]]*#/d' -e '/^[[:space:]]*$$/d' test/sql/manifest.txt | sort), which initially reported one missing entry.
• Added the missing test/sql/search_path_qualification_test.sql to test/sql/manifest.txt and re-ran the same comm/find/sed validation, which returned no missing files (guard logic passes).

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Codex Task