feat(lsp): add Metals LSP support for Scala

[YichenLi00]

Summary

• Add Metals LSP support for Scala files (.scala, .sbt, .sc)

Closes#10685

Implementation Details

Aspect	Decision
File extensions	.scala, .sbt, .sc
Project detection	build.sbt, build.sc, .scala-build, project/build.properties
Installation	Auto-download Coursier, then cs install metals
Platforms	macOS, Linux, Windows (x64 and arm64)
LSP command	metals (no arguments needed)
Initialization options	statusBarProvider: "log-message", isHttpEnabled: true

Why these decisions?

1. Coursier over direct binary download - Metals is distributed via Coursier (the Scala artifact manager). This is the official installation method and handles JVM version compatibility automatically.

2. Auto-download Coursier if missing - Follows existing patterns (gopls via go install, csharp-ls via dotnet tool install). Coursier binaries are available for all major platforms.

3. Java 11+ requirement - Metals requires Java 11 or later. The implementation checks for Java availability before attempting installation.

4. Project detection files - Covers SBT (build.sbt), Mill (build.sc), Scala CLI (.scala-build), and SBT version file (project/build.properties).

Files Changed

• packages/opencode/src/lsp/language.ts - Add .sbt and .sc extensions
• packages/opencode/src/lsp/server.ts - Add Metals LSP server
• packages/web/src/content/docs/lsp.mdx - Update documentation

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[YichenLi00]

The failing test (tool.registry) is unrelated to this PR - tracked in #12270. Will rebase once that's merged.

[YichenLi00]

@thdxr This implements #10685. Ready for review.

[YichenLi00]

@thdxr Rebased, CI passing now. Ready for review.

[majk-p]

@thdxr can you please have a look? 🙏

[cyber-barrista]

Any progress on the review?

[BugNotFoundX]

Any progress on the review? As a user of Scala, I really need this feature.

[rekram1-node]

lookin

[rekram1-node]

/review

[rekram1-node]

Are you sure this root detection works for sbt projects that only have project/build.properties? NearestRoot returns the matched file’s parent directory, so wouldn’t that make Metals start in /project instead of the actual project root?

[alexdupre]

If that is the behavior, then I don't think it'll work, but a scala project with only a "project/build.properties" without a build file is quite rare nowadays. Checking for just the "project" directory is likely too lax, so I think it'd be safe to simply remove the last element from the list.

[majk-p]

I think this list should also include build.millmill builds

[rekram1-node]

ignore current test failure, flaky test.