feat(mcp): every refused query records the guard rule that refused it (#1231)

[ZhuchkaTriplesix]

Refs #1231

What was wrong

McpSqlGuard returned only a message for a refused query. The activity log kept that text, so a refusal could be read but not grouped by the rule that caught it.

Changes

• McpSqlGuard.refusal returns a McpSqlRefusal with a stable rule id and the message. The nine refusals have ids: empty_query, single_statement, mysql_executable_comment, sqlite_pragma, read_only_only, function_not_allowed, explain_analyze_or_write, data_modifying, select_into_or_lock. check returns the same message as before.
• The id travels with the tool error (McpToolException.rule) to the call record and into the activity entry.
• mcp_activity gets a refusal_rule column (schema v12). Existing databases get it through an ALTER TABLE; rows written before it read as no rule.
• Tests: each rule id has an example; a query that may run has no refusal; the message is unchanged; the activity entry round-trips the rule, and an old row reads as no rule.

Not in this PR

• The argument-placement cases of the security suite (the second half of feat(mcp): refusals carry a rule id in the audit log; argument-placement cases in the security suite #1231).
• The rule ids are not shown in the Settings activity list yet.

Not verified locally

Tests were not run locally, per the project rule.