MongoDB: enforce Safe Mode reads in the plugin host, not only in the classifier #3293

Safe Mode and read-only external clients trust the MongoDB classifier's read verdict since #3292. The verdict covers the statement text, not the shell it runs in.

• The plugin keeps one JavaScript context per connection (MongoScriptRuntime).
• A statement Safe Mode allowed as a write can redefine a prelude function, for example DBCollection.prototype.find = DBCollection.prototype.drop.
• A later db.users.find({}) then passes as a read and drops the collection, at Read-Only or for a read-only MCP token, until reconnect.

Fix: carry read-only intent to the driver (a PluginKit addition with a default) and have MongoScriptHost.perform refuse write ops and $out/$merge pipelines for a statement sent as a read. Needs a plugin-mongodb release.