[ZhuchkaTriplesix]

Closes #1776 (the rest of it; the socket host, controller, access store and guard items went in #1802).

• find sends maxTimeMS (the MCP timeout) with the find command, count adds maxTimeMS to the count command. The server now stops a heavy $regex or $expr itself; before, the Dart future was dropped and the session closed, but the query could keep running. The find command needs MongoDB 3.6+ (OP_MSG); older servers get the plain find as before.
• McpMongoService.maxTotalChars (200000): the documents of a result together, as JSON, are capped; more are cut and the result is marked truncated. Each field was already capped at 4096 characters, but 100 documents of many such fields were not.
• Tests: the timeout reaches the session for find and count; a small total cap cuts the result and marks it truncated; a result under the cap is returned whole.

Not done: masking the connection's own secrets for non-tool exceptions in _guarded; the redaction by pattern (URL user info, password=, keys) still applies there. Not run locally (the analyzer is clean on the changed files), CI will run the tests.