Add cached-token telemetry to align with Copilot CLI

[Patrick McGannon (pmcgannon22)]

Problem

Cached-token telemetry in Copilot Chat was incomplete and did not fully align with Copilot CLI.

Fix

Propagate cached token usage through the shared telemetry pipeline and emit both cache-read and cache-creation token signals in OTel.

Changes

• add cache_creation_input_tokens to shared usage types
• propagate cache creation tokens through the Anthropic messages API path
• record cache read and cache creation token usage in OTel metrics
• add corresponding OTel span attributes
• preserve zero-valued cached token fields across GitHub, Anthropic BYOK, and Gemini BYOK paths
• add tests for the new token types

[Patrick McGannon (pmcgannon22)]

Patrick McGannon (@pmcgannon22) please read the following Contributor License Agreement(CLA). If you agree with the CLA, please reply with the following information.

@microsoft-github-policy-service agree [company="{your company}"]

Options:

• (default - no company specified) I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer.

@microsoft-github-policy-service agree

• (when company given) I am making Submissions in the course of work for my employer (or my employer has intellectual property rights in my Submissions by contract or applicable law). I have permission from my employer to make Submissions and enter into this Agreement on behalf of my employer. By signing below, the defined term “You” includes me and my employer.

@microsoft-github-policy-service agree company="Microsoft"

Contributor License Agreement

@microsoft-github-policy-service agree

[Alexandru Dima (alexdima)]

Thanks for the contribution! This repository has been archived because the project has moved into the main VS Code repository.

Could you please reopen/recreate this PR against:
https://github.com/microsoft/vscode/tree/main/extensions/copilot

We’ll continue reviewing contributions there. Thanks!