chore: Re-add sbt-scalafmt to the sbt 2.x plugin builds

[xerial]

Why

While sbt-uni and sbt-uni-playwright were pinned to sbt 2.0.0-RC10, sbt-scalafmt (which needs ≥ 2.0.0-RC11) had to be left out. Now that #624 moves them to sbt 2.0.1, the formatter can be re-added — restoring the same scalafmtCheckAll enforcement the main build already has.

Stacked on #624 (base = chore/bump-sibling-sbt-201). Review/merge that first; this diff is scalafmt-only on top of it.

What

• Add addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.6.1") (the version the main build uses) to sbt-uni and sbt-uni-playwright metabuilds.
• Add a local .scalafmt.conf to sbt-uni (it had none — its sources happened to already conform to the mirrored repo-root style).
• Reformat the 7 existing sbt-uni-playwright sources to that style (they were written before the formatter was available; formatting-only, still compiles).
• Enforce in CI: the test_sbt_plugin and test_sbt_uni_playwright jobs now run scalafmtCheckAll before scripted.

Validation

scalafmtCheckAll passes for both builds on sbt 2.0.1, and both still compile.

Follow-up: sbt-uni-crossproject (added in #623) gets the same treatment in that PR, so all three sbt 2.x builds end up consistent.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request integrates the sbt-scalafmt plugin and reformats the codebase according to the newly added .scalafmt.conf configuration. The review feedback identifies a potential resource leak in PlaywrightEngine.scala where an exception thrown during streams.close() in the finally block could prevent subsequent cleanups of the browser session and temporary files.

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[gemini-code-assist]

If streams.close() throws an exception, the subsequent critical cleanups (session.close() and materializer.close()) will be skipped. This can lead to leaked browser processes (Playwright) and leftover temporary files. Wrapping streams.close() in a try-catch block ensures that browser shutdown and file cleanup always execute.

      if streams != null then
        try streams.close()
        catch case NonFatal(_) => ()