🧹 fix: Prefer Structured Metadata Over Consumed Splat Args in Traversal

`debugTraverse` previously read `metadata[Symbol.for('splat')][0]` first
and only fell back to the structured metadata object. When a caller uses
printf interpolation alongside a metadata object — for example
`logger.warn('failed for %s', tenant, { provider })` — winston leaves the
*consumed* positional arg (`tenant`) in `SPLAT[0]` after interpolation.
The formatter would then append the tenant a second time and skip the
real metadata, regressing debug-file and `DEBUG_CONSOLE` output quality
now that warn/error share this path.

Prefer the structured metadata object (via `extractMetaObject`) and only
fall back to `SPLAT[0]` when there's nothing else, so the surviving log
line surfaces the actual key/value pairs regardless of call shape.

Reviewed-by: Codex (P2 finding on PR #12737, commit 1e43d636).

api/config/__tests__/parsers.spec.js

@@ -215,4 +215,18 @@ describe('debugTraverse', () => {
     const out = runFormatter(buildInfo('debug', { someField: 'not-sensitive' }));
     expect(out).toContain('not-sensitive');
   });
+
+  it('prefers structured metadata over a consumed printf arg in SPLAT[0]', () => {
+    const info = {
+      level: 'warn',
+      message: 'failed for tenant-7',
+      timestamp: 'ts',
+      provider: 'openai',
+      [SPLAT_SYMBOL]: ['tenant-7', { provider: 'openai' }],
+    };
+    const out = runFormatter(info);
+    expect(out).toContain('openai');
+    const tenantMatches = out.match(/tenant-7/g) ?? [];
+    expect(tenantMatches.length).toBeLessThanOrEqual(1);
+  });
 });

api/config/parsers.js

@@ -186,7 +186,17 @@ const debugTraverse = winston.format.printf(({ level, message, timestamp, ...met
       return finalize(msg);
     }
 
-    const debugValue = metadata[SPLAT_SYMBOL]?.[0] ?? extractMetaObject(metadata);
+    /*
+     * Prefer the structured metadata object (which winston merges into info)
+     * over `SPLAT_SYMBOL[0]`. The first splat entry may be a *consumed*
+     * printf arg — e.g. `logger.warn('failed for %s', tenant, { provider })`
+     * leaves `tenant` in `SPLAT[0]` after interpolation, which would then
+     * be appended again to the line instead of surfacing the real metadata
+     * object `{ provider }`.
+     */
+    const extracted = extractMetaObject(metadata);
+    const splatFirst = metadata[SPLAT_SYMBOL]?.[0];
+    const debugValue = extracted ?? splatFirst;
 
     if (!debugValue) {
       return finalize(msg);