Fix plugin upgrade terminal verification

[chubes4]

Summary

• decode the first valid plugin-state JSON array after bounded WP-CLI stdout preambles
• reject output without a valid array or requested plugin object
• replace malformed inline case command substitution with valid active-status rendering
• cover preamble acceptance and malformed-payload refusal in the bounded upgrade harness

Closes #471.

Verification

• bash -n lib/plugin-upgrade.sh tests/plugin-upgrade-bounds.sh
• shellcheck lib/plugin-upgrade.sh tests/plugin-upgrade-bounds.sh
• bash tests/plugin-upgrade-bounds.sh
• real WordPress Studio no-op convergence: DMC 0.72.1 active, release pointer verified, PLUGIN_UPGRADE_RESULT=complete
• git diff --check

AI assistance

OpenCode with openai/gpt-5.6-sol reproduced the false deployment failure, implemented the parser and shell corrections, and ran deterministic plus real Studio verification under human direction.