requestDiagnostics in LSP client hangs if a diagnostic request promise rejects

Description

In packages/opencode/src/lsp/client.ts, requestDiagnostics only attaches a fulfillment handler to each request promise:

for (const request of requests) {
  request.then((result) => {
    results.push(result)
    pending -= 1
    ...
  })
}

The only caller passes promises from requestDiagnosticReport, whose internal .catch(() => null) guards the textDocument/diagnostic request itself but not the report processing that runs after it. If that processing ever throws (e.g. a server returns a malformed report), the promise rejects and:

1. pending never reaches 0, so if done(results) is never satisfied the returned promise never resolves — the diagnostics flow hangs
2. the rejection is unhandled

Proposed fix: attach a .catch that decrements pending and force-finishes once all requests have settled, treating the failed request as contributing no results.