fix: anthropic tool-call round-trip, cache zero-hit parity, round-robin load balancing

anthropic.lua v3.0.0:
- Issue 1: tool_result/tool_use round-trip
- Issue 3: thinking default OFF (opt-in via extra_body.thinking)
- Issue 4: tool_choice mapping
- Issue 5: collect_blocks preserves unknown part types
- message_stop no longer emits done=true (was overwriting tool_calls finish_reason)
- cache_read_input_tokens normalized even at 0

gemini.lua:
- transform_response was missing cachedContentTokenCount

openai.lua (Issue 6):
- transform_error handles flat envelopes, nginx HTML, bare text

chat.go mergeUsage:
- Keep PromptTokensDetails even when CachedTokens=0

scheduler.go:
- Remove sort.SliceStable by Pref; round-robin cursor is the only LB mechanism

provider.go ModelAvailable:
- Also check Pref() > prefMin, persistently failing slots exit cands

presets.go:
- 17 built-in source templates

Tests: 6 new test functions, 2 updated for new semantics

internal/lua/adapters/openai.lua

@@ -136,13 +136,43 @@ end
 
 -- 错误收敛：标准 OpenAI 信封 {error:{message,...}}
 function adapter.transform_error(status, body)
+    -- Non-JSON body (nginx HTML error pages, plain text): extract a short
+    -- human-readable reason instead of letting the raw body reach the log.
     local ok, resp = pcall(json.decode, body)
-    if not ok or type(resp) ~= "table" then return nil end
+    if not ok or type(resp) ~= "table" then
+        -- HTML error page: pull the <title> text (e.g. "413 Request Entity Too Large")
+        local title = string.match(body or "", "<title>(.-)</title>")
+        if title and title ~= "" then return title end
+        -- Bare text: first non-empty line, capped
+        local line = string.match(body or "", "^%s*([^\r\n]+)")
+        if line and line ~= "" and not string.match(line, "^<") then
+            return string.sub(line, 1, 200)
+        end
+        return nil
+    end
+
+    -- Standard OpenAI envelope: {error:{message,...}} or {error:"..."}
     local e = resp.error
     if type(e) == "table" and type(e.message) == "string" then
         return e.message
     end
     if type(e) == "string" then return e end
+
+    -- Flat envelope used by many OpenAI-compatible gateways:
+    --   {"code":20012,"message":"Model does not exist..."}
+    --   {"code":"INVALID_API_KEY","message":"Invalid API key"}
+    if type(resp.message) == "string" and resp.message ~= "" then
+        if resp.code ~= nil then
+            return tostring(resp.code) .. ": " .. resp.message
+        end
+        return resp.message
+    end
+
+    -- Some gateways use {detail:"..."} (FastAPI style)
+    if type(resp.detail) == "string" and resp.detail ~= "" then
+        return resp.detail
+    end
+
     return nil
 end