fix: pass onWarn to viteResolvePlugin in build mode

[toshetah]

Fixes the Rolldown NAPI panic that crashes vite build when builtin:vite-resolve needs to emit a warning.

Problem

In build mode, viteResolvePlugin was not passed an onWarn callback. When the native builtin:vite-resolve NAPI plugin encountered a condition requiring a warning, it fell back to calling this.warn() on its Rust plugin context — which is marked unimplemented!() in Rolldown, causing a hard panic:

thread 'rolldown-worker' panicked at crates/rolldown_plugin/src/plugin_context/plugin_context.rs:
not implemented: Can't call warn on PluginContext::Napi

The onWarn callback was already provided in serve mode but was missing from the build mode branch.

Fix

Remove the command === 'serve' conditional so onWarn is passed unconditionally, routing warnings through Vite's logger in both modes.

Tests

A test is not included because the panic occurs inside Rolldown's native binary and cannot be reproduced in Vite's JS test suite. The fix is structural — onWarn was already exercised in serve mode; this change makes it unconditional.

Alternatives explored

• Suppressing the warning at the Rolldown level via rolldownOptions.checks flags — those are JS-level config, the Rust panic fires before they are consulted
• Using createBuilder with useLegacyBuilder: true — controls the Environment API shape only, Rolldown is still used either way

[toshetah]

The three failing checks are pre-existing CI issues unrelated to this change:

• node-20: pnpm@10.33.4 panics with ERR_UNKNOWN_BUILTIN_MODULE on Node.js 20 (infrastructure issue, fails before install)
• macos-latest: same "Test build" failure exists on main prior to this PR
• zizmor: pre-existing workflow security scan failure on main

The code change itself is a one-line fix removing a serve-mode guard from the onWarn callback.