feat: support Priority Processing (service_tier) and Cost Tracking (cost_in_usd_ticks)

[hrco]

Missing recent xAI API capabilities

Two API features shipped in 2026 that xBridge doesn't expose:

1. Priority Processing (June 2026)

xAI now supports service_tier: "priority" for higher scheduling priority on text, image, and video inference endpoints. The response's service_tier field reports the tier applied.

Location to add: make_grok_request(), make_image_request(), make_video_request() should accept an optional service_tier parameter.

Tool changes needed:

• Add service_tier parameter (enum: "auto", "priority") to grok-chat, grok-web-search, grok-x-search, grok-image-generate, grok-image-edit, grok-video-generate
• Pass it through to the API payloads
• Surface service_tier from response in the result text

2. Cost Tracking (April 2026)

Every API response now includes cost_in_usd_ticks in the usage object. xBridge currently ignores this.

Location to add: extract_response_text() or individual handlers should parse response.usage.cost_in_usd_ticks and append the cost to the returned text.

Tool changes needed:

• Extract cost_in_usd_ticks from API responses (text, image, video)
• Append cost info to the result text, e.g. "Cost: $0.0015"
• Update handle_grok_models() / handle_image_models() to mention cost tracking is available

Acceptance criteria

• service_tier parameter added to grok-chat, grok-web-search, grok-x-search tool schemas
• service_tier passed in API payload for text requests via make_grok_request()
• service_tier parameter added to image/video generation tools
• service_tier passed in image/video API payloads
• Response service_tier surfaced in result text
• cost_in_usd_ticks extracted from all response types
• Cost appended to result text output
• Tests for new parameters
• docs/wiki/Tools-Reference.md updated