Add coverage for post-tag-version-bump (pgxntool issue #20)

[jnasbyupgrade]

Summary

Adds BATS coverage for Postgres-Extensions/pgxntool#95, which adds make post-tag-version-bump (bumps default_version to a placeholder alias after a release, via new bump-default-version.sh) plus PGXNTOOL_ENABLE_POST_TAG_VERSION_BUMP/PGXNTOOL_POST_TAG_VERSION.

New test/standard/tag-version-bump.bats:

• Standalone script-logic tests for bump-default-version.sh (quoted styles, trailing comments, multiple control files, error cases) -- no make, no foundation, no PostgreSQL
• make -n dry-run coverage of post-tag-version-bump's enable/disable/value override wiring
• Stub-based coverage (_POST_TAG_VERSION_BUMP_SCRIPT) proving the script is genuinely invoked or skipped, without depending on its real behavior
• One real end-to-end test proving default_version actually gets bumped and the placeholder's SQL file gets generated on the next build

Also adds pgxntool/bump-default-version.sh to the exact distribution manifest (test/lib/dist-expected-files.txt).

Test plan

• test/standard/tag-version-bump.bats (8/8 passed)
• Full test-all suite (256/256 passed, 0 skipped)

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