test(mcp): security suite for the MCP server

[ZhuchkaTriplesix]

Security suite for the MCP epic #1133, with two hardening fixes it turned up.

Fixes

• Credentials in error messages. Driver errors can quote a connection string, an option list or a key. Every tool error (and internal errors) now goes through McpRedaction: the connection's own secrets when loaded (password, connection string, SSH password / key / passphrase), scheme://user:pass@ user info, password= / pwd= / passphrase= / token= / api_key= values and PEM private keys are masked. Also applied when creating the delegate fails.
• MySQL executable comments. MySQL / MariaDB run the body of /*! ... */ and /*M! ... */, which the comment stripper hid from the guard. For MySQL connections such comments are now refused.

Tests (test/core/mcp/mcp_security_test.dart)

• ~70 write forms across Postgres / MySQL / SQLite are refused by McpSqlGuard (DML, DDL, GRANT, COPY, CALL, DO, PREPARE/EXECUTE, SET / RESET / BEGIN READ WRITE, NOTIFY, stacked statements, data-modifying CTEs, EXPLAIN ANALYZE, SELECT INTO / INTO OUTFILE, row locks, server functions, comment-split keywords, executable comments, ATTACH, writable PRAGMAs, load_extension ...), and none of them reaches the delegate through run_query or explain_query.
• The production delegates are always created with isReadOnly: true (layer 2).
• Every tool and the schema:// resource, called through a real MCP client, never return password, SSH password / key / passphrase, host, user or file path: in normal results, in driver errors that quote all of them, and when delegate creation fails.
• A connection that is not shared is invisible to every tool and the resource, and no delegate is ever created for it.
• McpRedaction unit tests.

Already covered elsewhere and not duplicated: token refusal and 0600 / 0700 permissions (mcp_server_test.dart), timeout / row limit / cell truncation (mcp_query_service_test.dart), the database refusing a write on the real SQLite MCP session (mcp_sqlite_readonly_test.dart).

Not done: a live Postgres / MySQL read-only check; CI has no database services. The session flags themselves are covered by the pool tests.

Written without running locally; CI is the first run.

Closes #1138