genai: seed MCP and GenAI global-variable defaults on plugin initialization

[renecannao]

Context

Investigation of #6083 found that a fresh GenAI plugin load does not seed its owned mcp-* and genai-* rows in global_variables.

The AI setup runs statements such as SET mcp-enabled=true. For these plugin variables, SET becomes an UPDATE; when the row is absent it affects zero rows without establishing the requested value. In CI-ai-g2, the setup attempts to enable MCP repeatedly, while the plugin continues to report that MCP is disabled.

CI-ai-g1 can appear to recover only because an earlier test performs a save operation that happens to seed variables midway through the group. This makes later tests order-dependent.

Required behavior

A freshly initialized GenAI plugin must expose a complete, canonical set of MCP and GenAI variable rows with defaults before callers attempt to update them.

Acceptance criteria

• Fresh plugin initialization creates every supported mcp-* and genai-* row exactly once.
• SET mcp-enabled=true on a fresh instance updates an existing row and can start MCP after the documented load operation.
• Reinitialization does not overwrite persisted operator values.
• AI tests pass independently and in a different execution order.
• A regression test covers empty-database initialization and restart/persistence behavior.

Parent: #6098
Observed in: #6083
Related documentation gap: #5731

[coderabbitai]

🔗 Related PRs

#5461 - Migrate AI TAP group to Unified CI Infrastructure pattern [merged]
#5643 - fix(mysqlx): sync empty source tables to overwrite stale rows [merged]
#5700 - fix(test): test_mysqlx_plugin_load-t needs Phase B between load and init [merged]
#6097 - fix(ci): hand off GenAI plugin to AI shards [open]

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