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/chat.go

@@ -575,8 +575,14 @@ func recordChatUsage(rec *Req, req *types.ChatRequest, resp *types.UnifiedRespon
 	// read it back so the request record carries the hit/miss split.
 	if d := resp.TokenUsage.PromptTokensDetails; d != nil && d.CachedTokens > 0 {
 		rec.CacheHit = int64(d.CachedTokens)
+	}
+	if resp.TokenUsage.PromptTokensDetails != nil {
+		// Upstream reported cache details (even a 0 hit) — tag the row so
+		// the UI can show 0% rather than “—”.
+		rec.CacheReported = true
 	} else if resp.TokenUsage.PromptCacheHit > 0 {
 		rec.CacheHit = int64(resp.TokenUsage.PromptCacheHit)
+		rec.CacheReported = true
 	}
 	if resp.TokenUsage.PromptCacheMiss > 0 {
 		rec.CacheMiss = int64(resp.TokenUsage.PromptCacheMiss)
@@ -761,10 +767,14 @@ func (g *Gateway) pumpStream(w http.ResponseWriter, rec *Req, chunks <-chan type
 		tut = lastUsage
 		// Write the upstream's cache accounting back onto the request record
 		// so the audit trail carries the hit/miss split for streaming too.
-		if d := tut.PromptTokensDetails; d != nil && d.CachedTokens > 0 {
-			rec.CacheHit = int64(d.CachedTokens)
+		if d := tut.PromptTokensDetails; d != nil {
+			rec.CacheReported = true
+			if d.CachedTokens > 0 {
+				rec.CacheHit = int64(d.CachedTokens)
+			}
 		} else if tut.PromptCacheHit > 0 {
 			rec.CacheHit = int64(tut.PromptCacheHit)
+			rec.CacheReported = true
 		}
 		if tut.PromptCacheMiss > 0 {
 			rec.CacheMiss = int64(tut.PromptCacheMiss)