feat(mcp): add cluster filter to list_alert_rules (#4288)

[AvijitBhattacharjee]

Title: feat(mcp): add cluster and cluster_match filter to list_alert_rules

Closes #4288
Summary
Adds optional cluster and cluster_match parameters to the list_alert_rules MCP tool, allowing users to scope results to a specific cluster instead of always returning every rule from alert_rules.yml and ems_alert_rules.yml.
Changes
mcp/pkg/mcptypes/types.go

Added ListAlertRulesRequest struct with cluster (exact match) and cluster_match (regex) fields, consistent with GetActiveAlertsRequest and InfrastructureHealthRequest.

mcp/cmd/server/main.go

Updated ListAlertRules handler signature from struct{} to mcptypes.ListAlertRulesRequest
Added input validation (rejects " and \ in cluster values) matching the pattern used by GetActiveAlerts and InfrastructureHealth
Added filterRulesByCluster — filters []rules.RuleInfo by checking the rule's expr (substring match) and labels["cluster"] (exact or regex match)
Added ruleMatchesCluster — core matching logic handling exact cluster name and compiled regex cases
Added filter annotation in response output when filtering is active
Updated Total Rules count to reflect filtered results

mcp/cmd/descriptions/descriptions.go

Added ListAlertRulesDesc constant with full documentation of the new parameters, replacing the previous inline string in createMCPServer()

[cla-bot]

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[cla-bot]

Thank you for your interest in contributing to the Harvest project! We require contributors to sign our Corporate contributor license agreement (CCLA), and we don"t have the user(s) @AvijitBhattacharjee on file. In order for us to review and merge your code, please follow the instructions in step 6 of creating a pull request.
After signing the CCLA, you can ask us to recheck this PR by posting @cla-bot check as a comment to the PR.

[rahulguptajss]

@AvijitBhattacharjee Thanks for the contribution. Could you refill the CCLA form as your username in the filled form has a space as Avijit Bhattacharjee. It should be AvijitBhattacharjee. Once you do that, we can review the PR. Thanks.