fix(opencode): catch completed MCP browser launcher failures

[opencode-agent]

Summary

• Check the launcher's existing exit code as well as future exit events. open@11 can resolve only after the PowerShell launcher closes on Windows/WSL, so registering an exit listener alone can miss a nonzero exit and suppress BrowserOpenFailed.
• Reproduce the early-exit ordering with a real child process: a nonzero exit reports failure, while zero remains successful. No browser-opening behavior or OAuth decision changes.

Verification

• bun test test/mcp/browser.test.ts test/mcp/oauth-browser.test.ts test/mcp/oauth-provider.test.ts --timeout 30000 (18 passed)
• Scoped Prettier and oxlint (passed); git diff --check (passed)
• bun typecheck was attempted on the audit Host but its process ended without a result, so typecheck remains unverified here.
• Windows/WSL packaged-artifact validation is still needed; windows-dev was unreachable during this audit.

Related to #51414.

Requested by: @rekram1-node (Aiden via Slack)