ci: gate Claude Code install to Linux server-tests

Installing claude-code globally on the Windows server-tests runner
exposed 12 pre-existing Windows-specific test failures that were
silently skipped before (rmdir EBUSY during cleanup, .CMD spawn
EINVAL via the SDK, plus the launch-regression test that asserts
claude is *not* on PATH).

The Linux e2e tests (model-catalog, live-preferences, models) are
the only ones that needed the binary to flip isAvailable() true;
none of those run on Windows. Skip the install on Windows so those
Windows compatibility gaps stay surfaced separately and don't block
the SDK upgrade.

.github/workflows/ci.yml

@@ -88,7 +88,8 @@ jobs:
       - name: Install dependencies
         run: npm install
 
-      - name: Install Claude Code CLI for provider tests
+      - name: Install Claude Code CLI for provider tests (Linux only)
+        if: runner.os == 'Linux'
         run: npm install -g @anthropic-ai/claude-code
 
       - name: Build highlight dependency