⚡ perf: cache resolved theme in Theme Resolver

[lbajsarowicz]

Summary

• Cache resolved theme per area in Theme\Resolver::get() to avoid redundant theme collection loading within the same request.
• Uses sentinel boolean pattern ($isThemeResolved[$area]) to correctly cache null results without re-querying.
• Each area (frontend, adminhtml) is cached independently.

Performance Impact

Metric	Before	After
Theme collection queries	1× per get() call on cache miss	1× per area per request
Null handling	N/A	Correctly cached via sentinel flag

Test Plan

• Unit test verifies repeated get() calls return cached theme without re-resolving
• Unit test verifies null theme result is cached (sentinel pattern, not re-fetched)
• PHPCS clean (requires CI)
• PHPStan clean (requires CI)

Ref: #40721

⭐ Support my work

Do you like the fix? Remember to react with "👍🏻" to get it merged faster,
Then Sponsor me on Github so I can spend more time on fixing issues like this one.

Learn more at https://github.com/sponsors/lbajsarowicz

[m2-assistant]

Hi @lbajsarowicz. Thank you for your contribution!
Here are some useful tips on how you can test your changes using Magento test environment.
❗ Automated tests can be triggered manually with an appropriate comment:

• @magento run all tests - run or re-run all required tests against the PR changes
• @magento run <test-build(s)> - run or re-run specific test build(s)
  For example: @magento run Unit Tests

<test-build(s)> is a comma-separated list of build names.

Allowed build names are:

1. Database Compare
2. Functional Tests CE
3. Functional Tests EE
4. Functional Tests B2B
5. Integration Tests
6. Magento Health Index
7. Sample Data Tests CE
8. Sample Data Tests EE
9. Sample Data Tests B2B
10. Static Tests
11. Unit Tests
12. WebAPI Tests
13. Semantic Version Checker

You can find more information about the builds here
ℹ️ Run only required test builds during development. Run all test builds before sending your pull request for review.

---

For more details, review the Code Contributions documentation.
Join Magento Community Engineering Slack and ask your questions in #github channel.

[lbajsarowicz]

@magento run all tests