Add mpool_nonce_fix_auto_unblocks_pending as a regression test for main branch CI

[coderabbitai]

Summary

The test mpool_nonce_fix_auto_unblocks_pending was removed from the calibnet mpool integration test suite in #7229 because it was no longer needed as a pre-run step. However, it should be preserved and run as a regression test on main branch commits to ensure that the underlying mpool nonce-fix bug does not regress.

Required Changes

1. Restore (or adapt) the mpool_nonce_fix_auto_unblocks_pending test logic from src/dev/subcommands/tests_cmd/calibnet/mpool.rs (see fix(ci): allow concurrent wallet tests on CI #7229).
2. Wire it up to the CI pipeline so it runs automatically on commits to the main branch (e.g., in a nightly/regression workflow or the existing calibnet CI job restricted to main).
3. Ensure the test is clearly labelled/documented as a regression test for the mpool nonce-fix behaviour.

Rationale

The test validates that the mpool's automatic nonce-fix correctly unblocks pending messages. Dropping it entirely risks silent regression of the fix it was originally written to guard against.

References

• PR: fix(ci): allow concurrent wallet tests on CI #7229
• Comment: fix(ci): allow concurrent wallet tests on CI #7229 (comment)

/cc @sudo-shashank