Remove proxy-cached feature flag

[Copilot]

The proxy-cached experiment flag has been rolled out and is now always enabled. This PR removes the conditional logic as the proxy now defaults to cached mode.

Changes:

• Removed experiments['proxy-cached'] check from Updater.runUpdater()
• Removed cachedMode parameter from ProxyBuilder constructor
• Removed PROXY_CACHE environment variable (no longer needed as proxy defaults to cached mode)
• Updated integration tests to reflect simplified API

Before:

const cachedMode = this.details.experiments?.hasOwnProperty('proxy-cached') === true
const proxyBuilder = new ProxyBuilder(this.docker, this.proxyImage, cachedMode)

After:

const proxyBuilder = new ProxyBuilder(this.docker, this.proxyImage)
// Proxy now defaults to cached mode, no env var needed

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove the proxy-cached feature flag from dependabot-action</issue_title> <issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes github/dependabot-updates#12002

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