`sarif_list_rules`: add per-rule `resultCount` field

[data-douser]

Context

Sub-issue of #208 — Area 4: sarif_list_rules lacks per-rule result counts

Problem

The sarif_list_rules tool returns full rule metadata (description, help, tags, severity) for every rule in the SARIF file but does not include the count of results per rule. To determine which rules actually produced findings and how many, the SARIF file must be parsed separately.

Requirements

• Add a resultCount field to each rule in the sarif_list_rules response