Make matrix available to start-proxy action

[mbg]

This mirrors what we have for the other actions. Used by createStatusReportBase. I considered changing getRequiredInput("matrix") to getOptionalInput("matrix") there (and think that makes sense as a change), but thought it would be useful to have the matrix value in the start-proxy telemetry as well if it is available.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This change adds the matrix input parameter to the start-proxy action, making it consistent with other actions in the codebase. The matrix value is used by createStatusReportBase and will enable matrix information to be included in start-proxy telemetry.

Key Changes

• Added matrix input parameter to start-proxy/action.yml with default value from GitHub Actions matrix context
• Maintained consistency with existing action interfaces by making the parameter optional

[mbg]

You coordinated those reviews very well @henrymercer and @esbena 😄 within a few seconds of each other!