fix(lsp): surface gopls spawn failure reasons to LLM via tool output

[LinuxForYQH]

Issue for this PR

Closes #26865

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When gopls fails to start (e.g. Go not installed), Gopls.spawn() silently returns undefined. The .catch() in lsp.ts swallows it, so the LLM only sees "No LSP server available for this file type." — no clue what's actually wrong.

This PR:

• Makes Gopls.spawn() throw descriptive errors instead of silent return (2 cases: missing Go runtime, failed go install)
• Changes broken from Set<string> to Map<string, string> so failure reasons are stored alongside broken keys
• Adds failureReason(file) to the LSP service
• tool/lsp.ts queries failureReason() when LSP is unavailable and passes the actual error to the LLM

Now the LLM gets "Go runtime not found. Please install Go first — gopls will then be installed automatically." and can guide the user accordingly.

How did you verify your code works?

• npx tsc --noEmit --project packages/opencode/tsconfig.json — zero new compilation errors
• Traced the full call chain: server.ts throw → lsp.ts .catch() stores err.message in broken Map → tool/lsp.ts queries failureReason() → error surfaces to LLM

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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This PR doesn't have a linked issue. All PRs must reference an existing issue.

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[LinuxForYQH]

Rebased this PR onto the latest dev and cleaned it up to a single commit.\n\nLocal verification:\n- bun --cwd packages/opencode test --timeout 30000 test/tool/lsp.test.ts\n- bun run --cwd packages/opencode typecheck\n\nI also tried the broader related session tests, but they currently fail locally while starting the test server with Failed to start server. Is port 0 in use?; the LSP tool tests pass and the package typecheck passes.

[LinuxForYQH]

Hi @nexxeln, gentle ping on this one when you have a moment. The PR is now rebased onto the latest dev, cleaned up to a single commit, linked to #26865, and the GitHub checks are passing. Would appreciate a review whenever it fits your queue. Thanks!

[github-actions]

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