Improve global proxy tests

[mbg]

Improves our global-proxy e2e test. The existing test setup didn't actually verify that the proxy was used or force its usage. This PR improves the test by blocking HTTP(S) connections (for the standard ports). I have verified that this fails without the changes to api-client.ts from #4030 / this PR and succeeds with them.

Additionally, this PR slightly modifies makeProxyRequestOptions to always base the result on githubUtils.defaults.request and only overrides fetch. This should make it more robust if githubUtils.defaults.request includes other properties in the future or agent becomes more critical. I have also added two test cases there to check that the result either matches the default if no custom dispatcher is given, or matches the defaults with a different fetch otherwise. The actual proxy functionality is tested elsewhere.

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:

• Advanced setup - Impacts users who have custom CodeQL workflows.
• 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.
• Third-party analyses - The changes affect the upload-sarif action.

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.
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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]

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Pull request overview

Strengthens proxy validation and preserves GitHub API request defaults.

Changes:

• Forces HTTP(S) traffic through the proxy in the end-to-end test.
• Preserves default request options when customizing fetch.
• Adds unit coverage for both proxy configurations.

Show a summary per file

File	Description
src/api-client.ts	Preserves default request options.
src/api-client.test.ts	Tests proxy option construction.
pr-checks/checks/global-proxy.yml	Enforces proxy usage.
lib/entry-points.js	Generated artifact; excluded from review.
.github/workflows/__global-proxy.yml	Generated workflow; excluded from review.

Review details

Files excluded by content exclusion policy (2)

• .github/workflows/__global-proxy.yml
• lib/entry-points.js

• Files reviewed: 3/5 changed files
• Comments generated: 0
• Review effort level: Medium