Accessibility improvements to IDE component colours

[joshfarrant]

List of notable changes:

• Added note to IDE docs about tab contrast on glass variant
• Added 1px border to identify active tab and start/end of Copilot suggestion

What should reviewers focus on?

• Check that you're happy with the borders I've added
• Check that you're happy with the CSS variable names I've chosen
• Check that you're happy with the phrasing in the docs

Steps to test:

• 🔗 Link to Storybook
• 🔗 Link to Docs

Supporting resources (related issues, external links, etc):

• Closes https://github.com/github/primer/issues/3763
• Closes https://github.com/github/primer/issues/3764
• Closes https://github.com/github/primer/issues/3765

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Before	After

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9805140

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

🔍 Design token changes found

View CSS variable changes

+ --brand-IDE-default-editor-tab-borderColor: var(--base-color-scale-blue-5);

+ --brand-IDE-autoSuggest-borderColor: var(--base-color-scale-blue-5);

+ --brand-IDE-default-editor-tab-borderColor: var(--base-color-scale-blue-5);

+ --brand-IDE-autoSuggest-borderColor: var(--base-color-scale-blue-5);

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.