Add exposed filters to the usage log view (provider, model, period)

Problem/Motivation

The AI Usage Log at /admin/reports/ai-metering/log shows every recorded operation with no way to narrow the list. Finding records for a specific provider or model requires scrolling the full log or exporting and filtering offline.

Steps to reproduce

1. Navigate to /admin/reports/ai-metering/log.
2. Try to filter by provider or model.

Environment

• Drupal version: 10.x / 11.x
• ai_metering version: 1.0.0-alpha1
• AI provider module + version (e.g. ai_provider_anthropic 1.x): any
• AI feature module tested (e.g. ai_translate, ai_chatbot, ai_assistant_api, ai_summarize...): any

Proposed resolution

Add three exposed select-list filters to the ai_usage_log view: Provider, Model, and Period (combined Month/Year). Options for provider and model are populated dynamically from values already present in the log. The period filter defaults to the last 3 months.

Remaining tasks

• Create custom Views filter plugins for provider, model, and period.
• Register the filters in hook_views_data() and update the view config.
• Add a kernel test per filter.

User interface changes

The usage log page gains a filter form above the table.

API changes

N/A

Data model changes

N/A

AI assistance

Issue summary drafted with AI. Reviewed and approved by @codeitwisely.