Augment UsageDetails with cached / reasoning token counts

[stephentoub]

Cached tokens are currently reported by Anthropic, Gemini, OpenAI, and AWS. Reasoning tokens are currently reported by OpenAI and Gemini.

Closes #7101

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR adds first-class support for cached and reasoning token counts to the UsageDetails API by promoting them from AdditionalCounts to dedicated properties. These token counts are commonly reported by major AI providers including Anthropic, Gemini, OpenAI, and AWS, making this promotion valuable for API consistency and cost tracking.

Key Changes

• Added CachedInputTokenCount and ReasoningTokenCount nullable long properties to UsageDetails class
• Updated OpenAI client implementations to populate these properties directly from provider token usage details
• Migrated existing AdditionalCounts entries for cached/reasoning tokens to the new dedicated properties in OpenAI integrations
• Added comprehensive test coverage for the new properties including serialization, addition operations, and null handling

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/UsageDetails.cs	Added CachedInputTokenCount and ReasoningTokenCount properties with XML documentation, updated Add() method to sum them, and added debugger display support
src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json	Added API baseline entries marking both new properties as "Stable"
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs	Updated FromOpenAIUsage() to populate new properties from InputTokenDetails.CachedTokenCount and OutputTokenDetails.ReasoningTokenCount, removed these entries from AdditionalCounts
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs	Updated ToUsageDetails() to map InputTokenDetails.CachedTokenCount and OutputTokenDetails.ReasoningTokenCount to new properties
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/UsageDetailsTests.cs	New comprehensive test file covering constructor defaults, property roundtrips, Add() behavior with various combinations including nulls, and JSON serialization
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/UsageContentTests.cs	Updated serialization test to include CachedInputTokenCount and ReasoningTokenCount
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs	Updated multiple test methods to assert new properties are correctly populated and removed corresponding AdditionalCounts assertions
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs	Updated ResponseWithUsageDetails_ParsesTokenCounts test to verify new properties and assert AdditionalCounts is null when only cached/reasoning tokens are present