[Project Solar / Phase 1 / Engineering Follow-ups] Converted tests for HdsThemeSwitcher and HdsThemeContext to .gts format

[didoo]

📌 Summary

In previous PR #3428 we implemented the tests for the new HDS theming components in .ts format. Now that the main feature branch has been rebased on main we can leverage the work done by @shleewhite and convert these tests to .gts format as well.

🛠️ Detailed description

In this PR I have:

• converted tests for HdsThemeSwitcher and HdsThemeContext to .gts format

Note: the bulk of the work was done using Copilot in VSCode (GPT-5.2) and then tweaked.

🔗 External links

Jira ticket: https://hashicorp.atlassian.net/browse/HDS-5720

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👀 Component checklist

💬 Please consider using conventional comments when reviewing this PR.

📋 PCI review checklist

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've worked with GRC to document the impact of any changes to security controls.
  Examples of changes to controls include access controls, encryption, logging, etc.
• If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment.
  Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

[vercel]

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Project	Deployment	Review	Updated (UTC)
hds-showcase	Ready	Preview	Dec 19, 2025 8:13pm
hds-website	Ready	Preview	Dec 19, 2025 8:13pm

[Copilot]

Pull request overview

This PR converts integration tests for HdsThemeSwitcher and HdsThemeContext components from .ts format with hbs templates to the newer .gts format with inline template syntax. The conversion aligns with recent work to modernize the test suite and leverages Glimmer's template syntax improvements.

Key changes:

• Replaced hbs template strings with inline <template> syntax
• Updated component references from string-based to direct imports
• Converted test context variables to local scope variables with explicit TypeScript types

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
showcase/tests/integration/components/hds/theme-switcher/index-test.gts	Converted all test cases to use inline template syntax, replaced this.themingService with typed local variable, and removed ember-cli-htmlbars dependency
showcase/tests/integration/components/hds/theme-context/index-test.gts	Converted test templates to inline syntax, replaced this.set() calls with direct variable references in templates, and removed ember-cli-htmlbars dependency

[shleewhite]

Suggestion: We tend to avoid using before/after each now to keep things scoped to the individual test and only have access to the actual things you need for the individual test.

See this example for how we set up services:

design-system/showcase/tests/integration/helpers/hds-t-test.gts

Line 69 in 0ad049f

const intl = this.owner.lookup('service:intl');

[didoo]

done in d78a14b

(but, to me, it seems a lot of duplicated code for no reason, where the before/after I think are made exactly to avoid this)