Fix OpenCode custom command hangs (#1063)

* Fix OpenCode slash command turns

* ci: install opencode in server-tests so real OpenCode e2e runs

Server tests previously only installed the Claude Code CLI, so every
opencode-*.real.e2e.test.ts self-skipped on CI. The custom-command and
turn-lifecycle real tests cover the OpenCode session SSE rewrite, so
add opencode-ai globally and let them run. Big Pickle is free so no
provider secrets needed.

* Fix OpenCode unit tests exposed by enabling opencode in CI

server-tests previously skipped every OpenCodeAgentClient test because
opencode-ai was not installed. Now that CI runs them, four real issues
surface:

- Drop unused mkdirSync import (was used on a code path 1812b1489
  refactored away on main; the rebase merged imports without rechecking
  usage). Fixes the lint job.
- Fix slash-command-timeout test passing four ctor args instead of three,
  which silently dropped the TestOpenCodeRuntime and ran against the real
  opencode runtime.
- Relax the listModels contextWindowMaxTokens assertion. Some providers
  routed through OpenCode (OpenAI in particular) don't expose a numeric
  context window; assert the type only when the field is present.
- Swallow late notifySubscribers calls once close() flips the closed
  flag and clear subscribers up front. The session-lifetime SSE stream
  introduced by the prior commit means a session.error arriving after
  the owning test has moved on can rejectCompletion on a deferred no
  one is awaiting, surfacing as an unhandled rejection in a downstream
  test file.

.github/workflows/ci.yml

@@ -104,8 +104,8 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
-      - name: Install Claude Code CLI for provider tests
-        run: npm install -g @anthropic-ai/claude-code
+      - name: Install agent CLIs for provider tests
+        run: npm install -g @anthropic-ai/claude-code opencode-ai
 
       - name: Build highlight dependency
         run: npm run build --workspace=@getpaseo/highlight