Feat: 💰 Track cost in Usage value object

[vinitkadam03]

Description

Track cost in Usage value object. Some providers like OpenRouter return the cost of a request in their usage response. This adds a nullable float $cost property to Usage so that cost data can be captured when available, while remaining null for providers that don't report it.

Changes:

• Added ?float $cost = null to Usage value object and its toArray() output
• Updated Text\ResponseBuilder and Structured\ResponseBuilder to aggregate cost across steps
• Updated StreamState::addUsage() to accumulate cost during streaming
• Updated StreamEndEvent::toArray() to delegate to Usage::toArray() instead of inlining fields
• Updated OpenRouter Text, Structured, and Stream handlers to extract usage.cost from API responses
• Added UsageTest and updated existing tests for ResponseBuilders, StreamState, StreamEndEvent, and BroadcastAdapter