Issue #3615562: Price cache-write tokens, fix misleading help text

Jérôme Tchaniarequested to merge
issue/ai_metering-3615562:3615562-cache-write-token-cost into 1.0.x

Closes #3615562 https://www.drupal.org/project/ai_metering/issues/3615562

Adds cache_write_per_token to PricingSourceInterface, maps it in LiteLLMPricingSource (cache_creation_input_token_cost) and ModelsDotDevPricingSource (cost.cache_write), and extends TokenEstimator::calculateCost() to charge it. No caller currently supplies a non-zero count -- drupal/ai's TokenUsageDto has no cache-write field to read one from, tracked separately upstream (evidence-based, live-verified against real Anthropic and OpenAI calls: the gap is Anthropic-specific).

Also fixes the misleading cached_tokens Views help text, and makes AiPostGenerateSubscriber::extractTokenDetails() sweep the full TokenUsageDto instead of hardcoding reasoning/total, so a future DTO field is captured automatically.

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