Support linux-arm64 assets in start-proxy

[mbg]

We recently added support for CodeQL on the linux-arm64 platform. However, we did not update the start-proxy action, which downloads a platform-specific binary of the authentication proxy, accordingly. This PR makes a corresponding change to the start-action which uses the new getBundlePlatform function to determine the platform identifier. This effectively adds linux-arm64 as an option.

The remaining changes are refactors / tests.

Notes for reviewers

The current behaviour of start-proxy when run on linux-arm64 is to download the linux64 binary, which then fails to run because of the architecture mismatch.

We don't yet have linux-arm64 binaries of the proxy available in any CodeQL release, although there is a PR on dependabot/proxy to start building those.

Therefore, with this change on its own, start-proxy will still currently fail on linux-arm64 but because it cannot download the linux-arm64 asset from the CodeQL release.

I am rating this as "Low risk" because start-proxy is already failing as-is.

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.

Which use cases does this change impact?

Workflow types:

• Managed - Impacts users with dynamic workflows (Default Setup, Code Quality, ...).

Products:

• Code Scanning - The changes impact analyses when analysis-kinds: code-scanning.
• Code Quality - The changes impact analyses when analysis-kinds: code-quality.
• Other first-party - The changes impact other first-party analyses.

Environments:

• Dotcom - Impacts CodeQL workflows on github.com and/or GitHub Enterprise Cloud with Data Residency.
• GHES - Impacts CodeQL workflows on GitHub Enterprise Server.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).
• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

How will you know if something goes wrong after this change is released?

• Telemetry - I rely on existing telemetry or have made changes to the telemetry.
  • Dashboards - I will watch relevant dashboards for issues after the release. Consider whether this requires this change to be released at a particular time rather than as part of a regular release.
  • Alerts - New or existing monitors will trip if something goes wrong with this change.

Are there any special considerations for merging or releasing this change?

• Special considerations - See PR description regarding the availability of linux-arm64 binaries for the proxy. We can merge this PR anytime without those being available, since it won't make things worse (they are already failing).

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]

Warning

• Copilot's review of this pull request may be incomplete because some of the changed files are excluded by your Copilot content exclusion settings. See Excluding content from Copilot for details.

Copilot review overview

🟢 Approval recommended

The focused platform-selection change is consistent with existing helpers and adequately tested.

Review effort: Balanced
Findings: None

What changed in this PR

Adds Linux ARM64 asset selection to start-proxy using shared platform detection.

Changes:

• Adds platform and architecture to action state.
• Selects platform-specific proxy assets with Linux ARM64 support.
• Updates unit tests and test-state helpers.

File	Description
src/​action-common.ts	Adds runtime platform and architecture to base state.
src/​testing-utils.ts	Populates the new state fields in tests.
src/​start-proxy.ts	Selects proxy packages by bundle platform.
src/​start-proxy-action.ts	Passes action state into proxy resolution.
src/​start-proxy.test.ts	Updates and extends proxy platform tests.
lib/​entry-points.js	Generated bundle; excluded from review.

Files excluded by content exclusion policy (1)

• lib/entry-points.js

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[henrymercer]

This looks good. Once that linux-arm64 binary is available, consider adding a PR check that validates that the latest bundle contains start-proxy assets for each platform for which we have bundles. This could also live internally.