Add proxy_ca_certificate_file output to start-proxy action

[mbg]

I figured it would be useful to avoid duplicating this across different extractors that e.g. set SSL_CERT_FILE for relevant sub-processes. That's currently done by the C# extractor and looks like will be done by the Go extractor as well. By doing it in the action, we can avoid duplicating this process in those extractors.

Notes/questions for reviewers

• We might take this opportunity to add corresponding cleanup logic here as well.
• Currently, the implementation is such that we continue as before if writing the file fails, but we should probably just fail hard to guarantee that the file has been written if the proxy is initialised.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[Copilot]

Pull Request Overview

This PR adds an output for the proxy CA certificate file in the start-proxy action to avoid duplicating certificate handling in different extractors.

• Adds FS import and logic to write the CA certificate to a temporary file.
• Updates the ProxyConfig type to include an optional certificate file property.
• Mirrors the changes in both the TypeScript and JavaScript implementations.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/start-proxy-action.ts	Adds certificate file writing logic and exposes its file path output.
lib/start-proxy-action.js	Applies the same certificate file writing changes as in the TS file.

Comments suppressed due to low confidence (1)

lib/start-proxy-action.js:122

• The catch block only logs the error without failing the action. To guarantee the certificate file is properly written, consider rethrowing the error so that the process fails on write errors.

    } catch (error) {

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[Copilot]

The catch block only logs the error without failing the action. Given the PR description's goal to ensure the certificate file is successfully written, consider rethrowing the error to fail execution if the write fails.