Add optional usage quotas for Explain Error provider calls

[shenxianpeng]

Problem

Administrators may need to control plugin usage and provider cost, especially in shared Jenkins environments.

Goal

Allow optional quotas for real Explain Error provider calls.

Scope

V1 quota scope:

• controller-level quota only
• applies only to real provider calls
• does not apply to cache hits
• time-window based (for example hourly or daily)

V2 quota scope

• Folder-level quotas
• User-level quotas
• Provider billing reconciliation
• Automatic fallback to another model/provider

Proposed Behavior

When the configured quota is exceeded:

• reject the request before the provider call
• show a clear user-facing message
• record a quota_rejected usage event
• increment quota rejection metrics

Proposed Configuration

Add optional global settings such as:

• enable quota
• quota window
• max provider calls per window

Acceptance Criteria

• Requests are allowed until the configured quota is reached.
• Cache hits do not consume quota.
• Rejected requests do not call the provider.
• Quota rejections are visible in usage tracking and metrics.
• Existing behavior is unchanged when quotas are disabled.