test: verify open connections across DM8 HA takeover

[skhe]

Summary

• Add a repeatable real DM8 watcher takeover check for already-open Python autocommit and manual-transaction connections.
• Verify an interrupted manual transaction cannot report a successful commit or leak its uncommitted row after promotion.
• Verify the existing autocommit connection reaches the promoted primary and preserves exact DECIMAL(30,8) values before and after takeover.
• Record local Orb test evidence for takeover in both directions.

Verification

• Isolated ARM DM8 two-node watcher group and confirmation monitor in Orb: two successive automatic takeovers, both passed.
• Monitor recorded takeover by GRP453932_DW2 and then GRP453932_DW1.
• Both runs: held connection recovered with 0 visible errors; lost transaction commit raised; decimal values matched exactly.
• ruff check, ruff format --check, Python compile, and git diff --check passed.

The HA check remains local because GitHub-hosted runners cannot reach the user's Orb network. This PR adds a reproducible verifier and evidence without changing runtime behavior.