fix(tracing): avoid SetGlobalDefaultError panic when rolldown devtools is enabled (#1364)

## Summary

Fixes a panic that occurs when `build.rolldownOptions.devtools` is set in `vite.config.ts` (commonly via `@vitejs/devtools`):

```
Rolldown panicked. This is a bug in Rolldown, not your code.
thread '<unnamed>' panicked at tracing-subscriber-0.3.23/src/util.rs:94:14:
failed to set global default subscriber:
SetGlobalDefaultError("a global default trace dispatcher has already been set")
```

## Root cause

The vite-plus NAPI binary bundles `rolldown_binding` (including `rolldown_devtools`) into a single `.node` file. Two paths inside this same binary each try to install a global tracing subscriber:

1. **NAPI module load** → `vite_shared::init_tracing()` — previously installed a no-op subscriber (empty `Targets` filter) even when `VITE_LOG` was unset, occupying the global default slot.
2. **Build with devtools enabled** → `DebugTracer::init()` — tries to install its own subscriber and panics because the slot is already taken.

## Fix

- Skip subscriber installation entirely when `VITE_LOG` is not set, leaving the global default slot free for `DebugTracer::init()` (the common case — no one pays for tracing they didn't ask for).
- Use `.try_init().ok()` instead of `.init()` so a double-init still fails gracefully rather than panicking.

## Test plan
- [x] Reproduced the panic at `/tmp/repro-1356` with `@vitejs/devtools@0.1.13` + `build.rolldownOptions.devtools: {}`.
- [x] Rebuilt with `RELEASE_BUILD=1 pnpm bootstrap-cli`; `npx vp build` now completes successfully.
- [x] `VITE_LOG=debug vp build` still prints traces (subscriber is installed when requested).

## Related

Upstream rolldown also makes the same `.init()` → `.try_init()` change as defense-in-depth: rolldown/rolldown#9078. That upstream fix is not required for this patch — the vite-plus fix here is sufficient on its own.

Closes #1356

Author: fengmk2

crates/vite_shared/src/tracing.rs

@@ -1,6 +1,6 @@
 //! Tracing initialization for vite-plus
 
-use std::sync::OnceLock;
+use std::{str::FromStr, sync::OnceLock};
 
 use tracing_subscriber::{
     filter::{LevelFilter, Targets},
@@ -14,25 +14,28 @@ use crate::env_vars;
 /// Uses `OnceLock` to ensure tracing is only initialized once,
 /// even if called multiple times.
 ///
+/// Only sets the global default subscriber when `VITE_LOG` is set.
+/// When unset, the global default slot is left free so that other
+/// subscribers (e.g., rolldown devtools) can claim it without panicking.
+///
 /// # Environment Variables
 /// - `VITE_LOG`: Controls log filtering (e.g., "debug", "vite_task=trace")
 pub fn init_tracing() {
     static TRACING: OnceLock<()> = OnceLock::new();
     TRACING.get_or_init(|| {
+        let Ok(env_var) = std::env::var(env_vars::VITE_LOG) else {
+            return;
+        };
+
         tracing_subscriber::registry()
             .with(
-                std::env::var(env_vars::VITE_LOG)
-                    .map_or_else(
-                        |_| Targets::new(),
-                        |env_var| {
-                            use std::str::FromStr;
-                            Targets::from_str(&env_var).unwrap_or_default()
-                        },
-                    )
+                Targets::from_str(&env_var)
+                    .unwrap_or_default()
                     // disable brush-parser tracing
                     .with_targets([("tokenize", LevelFilter::OFF), ("parse", LevelFilter::OFF)]),
             )
             .with(tracing_subscriber::fmt::layer())
-            .init();
+            .try_init()
+            .ok();
     });
 }