docs(providers): restore the DeepSeek legacy reasoning-format contract

The openai-compat consolidation dropped the comment warning that DeepSeek needs
the legacy top-level "reasoning_effort" string form; without it a maintainer
could add openai.WithModernReasoningFormat() to the shared opts and silently
break DeepSeek thinking mode. Restore it at the shared opts choke point.

Also fix two stale pointers: glm/qwen config.yml cited
WithPreserveReasoningContent() "in glm.go/qwen.go", but it moved to
openaicompat.go during that same consolidation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/pkg/providers/glm/config.yml

@@ -10,7 +10,7 @@
 # on the standard API endpoint Z.AI defaults to clearing reasoning_content between
 # turns, which hurts agent loops with tool calls. Setting clear_thinking=false
 # preserves reasoning_content across turns, improving reasoning continuity and cache
-# hit rates. Required together with WithPreserveReasoningContent() in glm.go so that
+# hit rates. Required together with WithPreserveReasoningContent() in openaicompat.go so that
 # langchaingo serializes reasoning_content back to the API.
 #
 # Note: langchaingo's IsReasoningModel matches glm-4.5/4.6/4.7 prefixes and force-

backend/pkg/providers/openaicompat/openaicompat.go

@@ -60,6 +60,10 @@ func New(
 		openai.WithBaseURL(spec.ServerURL),
 		openai.WithHTTPClient(httpClient),
 	}
+	// Do NOT add openai.WithModernReasoningFormat() to these shared opts: DeepSeek's
+	// API requires the legacy top-level "reasoning_effort" string, which langchaingo
+	// emits only while ModernReasoningFormat is off (the default). The modern
+	// "reasoning":{} object form breaks DeepSeek thinking mode.
 	if spec.PreserveReasoning {
 		opts = append(opts, openai.WithPreserveReasoningContent())
 	}

backend/pkg/providers/qwen/config.yml

@@ -16,7 +16,7 @@
 #   - preserve_thinking=true: keeps reasoning_content from previous assistant turns
 #     in subsequent requests. Supported ONLY by qwen3.7-max and qwen3.6-plus families.
 #     Required for agent loops with tool calls to preserve reasoning continuity.
-#     Works together with WithPreserveReasoningContent() in qwen.go.
+#     Works together with WithPreserveReasoningContent() in openaicompat.go.
 #   - qwen3-coder-* (coder, installer) are NOT hybrid thinking models — no thinking
 #     control needed.