feat(prometheus): expose PromQL query as invokable provider method

[ykd007]

Problem

Closes #6475

Users want to use Keep's AI assistant to query Prometheus directly via PromQL. Currently PrometheusProvider has a private _query() method that isn't registered as an invokable method, so it doesn't appear in the AI agent's tool list or the Keep provider UI.

Solution

• Added PROVIDER_METHODS registration for a new execute_query method
• Added public execute_query(query: str) -> dict wrapper with friendly HTTP error messages
• Keep's ProviderMethodDTO auto-infers the query parameter from the function signature via reflection — no manual param spec needed

Now users can invoke PromQL queries directly from the Keep AI assistant or workflow actions.

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[ykd007]

I have read the CLA Document and I hereby sign the CLA