feat(types): pass through upstream cache tokens in TokenUsage

dsh displays cache-hit %, but llmsproxy dropped every upstream's cache
fields — deepseek prompt_cache_hit_tokens, OpenAI prompt_tokens_details.
cached_tokens, anthropic cache_read_input_tokens, gemini cachedContentTokenCount.

Changes:
- TokenUsage: add PromptTokensDetails (with CachedTokens) + PromptCacheHit/Miss
- MarshalJSON: emit prompt_tokens_details.cached_tokens (OpenAI v2 standard)
  and prompt_cache_hit/miss_tokens (DeepSeek legacy) — dsh reads the former
  first, falls back to the latter
- mergeUsage: preserve cache fields across stream chunks
- standardSSEChunk: parse the upstream raw prompt_tokens_details too
- deepseek.lua: forward prompt_cache_hit/miss_tokens + create
  prompt_tokens_details from them
- openai.lua: forward prompt_tokens_details.cached_tokens and legacy
  prompt_cache_hit/miss_tokens; normalize legacy hits into the standard
  object so dsh sees them regardless of upstream format
- anthropic.lua: map cache_read_input_tokens → prompt_tokens_details
- gemini.lua: map cachedContentTokenCount → prompt_tokens_details

internal/lua/adapters/anthropic.lua

@@ -82,6 +82,13 @@ function adapter.transform_response(raw_body)
         unified.token_usage.prompt = resp.usage.input_tokens or 0
         unified.token_usage.completion = resp.usage.output_tokens or 0
         unified.token_usage.total = (resp.usage.input_tokens or 0) + (resp.usage.output_tokens or 0)
+        -- Anthropic reports cache_read_input_tokens; normalize into
+        -- OpenAI-standard prompt_tokens_details.cached_tokens so clients
+        -- (dsh) see the cache hit count.
+        local cacheRead = resp.usage.cache_read_input_tokens or 0
+        if cacheRead > 0 then
+            unified.token_usage.prompt_tokens_details = { cached_tokens = cacheRead }
+        end
     end
 
     if resp.content and #resp.content > 0 then
@@ -107,6 +114,10 @@ function adapter.transform_stream_chunk(raw_chunk)
             local c = u.output_tokens or 0
             if p > 0 or c > 0 then
                 uses = { prompt = p, completion = c, total = p + c }
+                local cacheRead = u.cache_read_input_tokens or 0
+                if cacheRead > 0 then
+                    uses.prompt_tokens_details = { cached_tokens = cacheRead }
+                end
             end
         end
         if uses ~= nil then

internal/lua/adapters/deepseek.lua

@@ -49,6 +49,11 @@ function adapter.transform_response(raw_body)
         unified.token_usage.prompt = resp.usage.prompt_tokens or 0
         unified.token_usage.completion = resp.usage.completion_tokens or 0
         unified.token_usage.total = resp.usage.total_tokens or 0
+        unified.token_usage.prompt_cache_hit_tokens = resp.usage.prompt_cache_hit_tokens or 0
+        unified.token_usage.prompt_cache_miss_tokens = resp.usage.prompt_cache_miss_tokens or 0
+        if resp.usage.prompt_tokens_details and resp.usage.prompt_tokens_details.cached_tokens then
+            unified.token_usage.prompt_tokens_details = { cached_tokens = resp.usage.prompt_tokens_details.cached_tokens }
+        end
     end
 
     if type(resp.choices) == "table" and #resp.choices > 0 then
@@ -92,7 +97,12 @@ function adapter.transform_stream_chunk(raw_chunk)
             prompt = chunk.usage.prompt_tokens or chunk.usage.prompt or 0,
             completion = chunk.usage.completion_tokens or chunk.usage.completion or 0,
             total = chunk.usage.total_tokens or chunk.usage.total or 0,
+            prompt_cache_hit_tokens = chunk.usage.prompt_cache_hit_tokens or 0,
+            prompt_cache_miss_tokens = chunk.usage.prompt_cache_miss_tokens or 0,
         }
+        if chunk.usage.prompt_tokens_details and chunk.usage.prompt_tokens_details.cached_tokens then
+            uses.prompt_tokens_details = { cached_tokens = chunk.usage.prompt_tokens_details.cached_tokens }
+        end
     end
 
     if not chunk.choices or #chunk.choices == 0 then

internal/lua/adapters/gemini.lua

@@ -100,6 +100,10 @@ function adapter.transform_stream_chunk(raw_chunk)
         local t = chunk.usageMetadata.totalTokenCount or 0
         if p > 0 or c > 0 or t > 0 then
             uses = { prompt = p, completion = c, total = t }
+            local cacheRead = chunk.usageMetadata.cachedContentTokenCount or 0
+            if cacheRead > 0 then
+                uses.prompt_tokens_details = { cached_tokens = cacheRead }
+            end
         end
     end
 

internal/lua/adapters/openai.lua

@@ -33,6 +33,22 @@ function adapter.transform_response(raw_body)
         unified.token_usage.prompt = resp.usage.prompt_tokens or 0
         unified.token_usage.completion = resp.usage.completion_tokens or 0
         unified.token_usage.total = resp.usage.total_tokens or 0
+        -- Cache passthrough: OpenAI v2 prompt_tokens_details.cached_tokens
+        -- and DeepSeek-legacy prompt_cache_hit/miss_tokens. dsh reads
+        -- prompt_tokens_details.cached_tokens (falls back to the legacy
+        -- standalone field), so both shapes reach clients.
+        local hit = 0
+        if type(resp.usage.prompt_tokens_details) == "table" and (resp.usage.prompt_tokens_details.cached_tokens or 0) > 0 then
+            hit = resp.usage.prompt_tokens_details.cached_tokens
+            unified.token_usage.prompt_tokens_details = { cached_tokens = hit }
+        end
+        if (resp.usage.prompt_cache_hit_tokens or 0) > 0 then
+            unified.token_usage.prompt_cache_hit_tokens = resp.usage.prompt_cache_hit_tokens
+            unified.token_usage.prompt_cache_miss_tokens = resp.usage.prompt_cache_miss_tokens or 0
+            if hit == 0 then
+                unified.token_usage.prompt_tokens_details = { cached_tokens = resp.usage.prompt_cache_hit_tokens }
+            end
+        end
     end
 
     if type(resp.choices) == "table" and #resp.choices > 0 then
@@ -77,6 +93,13 @@ function adapter.transform_stream_chunk(raw_chunk)
             completion = chunk.usage.completion_tokens or chunk.usage.completion or 0,
             total = chunk.usage.total_tokens or chunk.usage.total or 0,
         }
+        if type(chunk.usage.prompt_tokens_details) == "table" and (chunk.usage.prompt_tokens_details.cached_tokens or 0) > 0 then
+            uses.prompt_tokens_details = { cached_tokens = chunk.usage.prompt_tokens_details.cached_tokens }
+        elseif (chunk.usage.prompt_cache_hit_tokens or 0) > 0 then
+            uses.prompt_cache_hit_tokens = chunk.usage.prompt_cache_hit_tokens
+            uses.prompt_cache_miss_tokens = chunk.usage.prompt_cache_miss_tokens or 0
+            uses.prompt_tokens_details = { cached_tokens = chunk.usage.prompt_cache_hit_tokens }
+        end
     end
 
     if not chunk.choices or #chunk.choices == 0 then