feat(ai): Rename deprecated span attrs to OTEL names and add granular cost fields

[vgrozdanic]

Summary

• Rename three SpanData usage fields to their new OTEL semantic convention names, keeping old names as legacy_alias for backward compatibility:
  • gen_ai.usage.input_tokens.cached → gen_ai.usage.cache_read.input_tokens
  • gen_ai.usage.input_tokens.cache_write → gen_ai.usage.cache_creation.input_tokens
  • gen_ai.usage.output_tokens.reasoning → gen_ai.usage.reasoning.output_tokens
• Add three new granular cost fields mirroring the usage token breakdown:
  • gen_ai.cost.cache_read.input_tokens
  • gen_ai.cost.cache_creation.input_tokens
  • gen_ai.cost.reasoning.output_tokens
• Update CalculatedCost to expose individual cost components
• Works in both span-first and transaction-spans paths

Dependencies

• Requires feat(ai): Add granular cost attributes for cache and reasoning sentry-conventions#461 to be merged and submodule updated

Test plan

• All existing AI cost calculation tests pass with updated snapshots
• Backward compat: old attribute names (e.g. gen_ai.usage.input_tokens.cached) still deserialize via legacy_alias
• New OTEL names (e.g. gen_ai.usage.cache_read.input_tokens) work correctly
• Granular cost fields are set for both SpanData (transaction path) and EAP (span-first path)
• Full cargo build and cargo test pass

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