fix(plugins): validate plugin command messages

[aiirvizionz]

Summary

• reject missing, blank, or non-string plugin command messages before plugin initialization
• trim valid command strings before dispatching to the plugin manager
• add focused route coverage for malformed JSON, non-string messages, and trimmed valid commands

Validation

• git diff --check
• node node_modules\vitest\vitest.mjs run --config $env:TEMP\qryptchat-vitest-minimal.config.mjs "src/app/api/plugins/execute/route.test.js" (3 tests)

Billing

• uGig billable PR candidate: expected invoice $0.50 if accepted/merged
• Not counted as revenue until paid/settled