feat: surface zero cache hits — distinguish 'missed' from 'not reported'

Live testing across the zen pool showed models report
prompt_tokens_details.cached_tokens even when the hit count is 0 (e.g.
nemotron-3-ultra-free returns cached_tokens:0, audio_tokens:0,
cache_write_tokens:0). The previous >0 guard dropped those objects, so a
cache-enabled upstream looked identical to one without cache support.

- types: PromptTokensDetails.CachedTokens always emitted (drop inner
  omitempty) so clients see cached_tokens:0 explicitly; dsh reads it as
  a 0% hit instead of 'no data'
- adapters (9): forward prompt_tokens_details whenever the upstream
  provides it (presence check instead of >0)
- Req: add cache_reported flag set when usage carried cache accounting;
  WebUI shows an amber 0% tag for reported-but-missed rows and keeps
  the em-dash only for sources that never report cache data

internal/gateway/ui/index.html

@@ -1990,10 +1990,11 @@
       // the upstream reported cache accounting, otherwise an em dash.
       function cacheCell(r) {
         const hit = r.cache_hit_tokens || 0;
-        if (!hit && !(r.cache_miss_tokens > 0)) return '<span class="muted">—</span>';
+        if (!hit && !(r.cache_miss_tokens > 0) && !r.cache_reported)
+          return '<span class="muted" title="上游未报告缓存数据">—</span>';
         const prompt = r.prompt_tokens || 0;
         const pct = prompt > 0 ? Math.round((hit * 100) / prompt) : 0;
-        return `<span class="tag ${pct >= 50 ? "tag-green" : pct > 0 ? "tag-amber" : ""}" title="命中 ${hit} / 未命中 ${r.cache_miss_tokens || 0}">${pct}%</span>`;
+        return `<span class="tag ${pct >= 50 ? "tag-green" : "tag-amber"}" title="命中 ${hit} / 未命中 ${r.cache_miss_tokens || 0}">${pct}%</span>`;
       }
       function showModelConfig(srcName, model) {
         const el = $("#conncfg");