feat: add results grouped by tags tab

[Lex-resource]

Closes #1108

Adds a dedicated Tags report tab that groups every tagged test result under each of its distinct tag values. This supports tag-based views such as Jira issue keys and test types without mixing untagged tests into the tree.

The change:

• generates data/tags.json from the existing tags extra block
• registers a Tags tree tab in the web UI
• adds focused aggregator and report-generation coverage

Validation:

• targeted Java tests (TagsTreePluginTest, ReportGeneratorTest)
• Spotless and Checkstyle
• frontend lint, i18n lint, typecheck, and build
• generated demo report visually verified

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Lex-resource]

The Verify type labels check needs a maintainer-applied type label; ype:new feature appears to match this PR. The forked Build and Validate Gradle Wrapper workflows are also awaiting maintainer approval. Local targeted Java tests, formatting/checkstyle, frontend lint/typecheck/build, and a generated demo report all pass.