feat(gateway): align body shape with real Claude Code CLI defaults

Three field-level alignments in normalizeClaudeOAuthRequestBody to
match real Claude Code CLI traffic byte-for-byte:

  1. temperature: previously deleted unconditionally; now passes
     through client value, defaults to 1 when absent (real CLI
     always sends temperature, default 1).

  2. max_tokens: defaults to 128000 when absent (real CLI default).

  3. context_management: when thinking.type is enabled/adaptive
     and the client did not provide context_management, inject
     {"edits":[{"type":"clear_thinking_20251015","keep":"all"}]}
     to mirror real CLI behavior.

tool_choice removal is unchanged (Claude Code OAuth credentials
do not allow client-supplied tool_choice).

Tests updated:
  - gateway_body_order_test.go: temperature/max_tokens are now
    expected in output; tool_choice still removed.
  - gateway_prompt_test.go: system array is now 2 blocks
    (billing + cc prompt), assertions adjusted.
  - gateway_anthropic_apikey_passthrough_test.go: same 2-block
    assertion.

backend/internal/service/gateway_service.go

@@ -1078,12 +1078,38 @@ func normalizeClaudeOAuthRequestBody(body []byte, modelID string, opts claudeOAu
 		}
 	}
 
-	if gjson.GetBytes(out, "temperature").Exists() {
-		if next, ok := deleteJSONPathBytes(out, "temperature"); ok {
+	// temperature：真实 Claude Code CLI 总是发送 temperature（默认 1，客户端可覆盖）。
+	// 之前的实现直接 delete 会导致 payload 缺字段，与真实 CLI 字节级不一致。
+	// 策略：客户端传了什么就透传；没传则补默认 1。
+	if !gjson.GetBytes(out, "temperature").Exists() {
+		if next, ok := setJSONValueBytes(out, "temperature", 1); ok {
 			out = next
 			modified = true
 		}
 	}
+
+	// max_tokens：真实 CLI 的默认值是 128000。缺失时补齐以对齐指纹。
+	if !gjson.GetBytes(out, "max_tokens").Exists() {
+		if next, ok := setJSONValueBytes(out, "max_tokens", 128000); ok {
+			out = next
+			modified = true
+		}
+	}
+
+	// context_management：thinking.type 为 enabled/adaptive 时，真实 CLI 会自动
+	// 附带 {"edits":[{"type":"clear_thinking_20251015","keep":"all"}]}。
+	// 客户端显式传了就透传；否则按 CLI 行为补齐。
+	if !gjson.GetBytes(out, "context_management").Exists() {
+		thinkingType := gjson.GetBytes(out, "thinking.type").String()
+		if thinkingType == "enabled" || thinkingType == "adaptive" {
+			const cmDefault = `{"edits":[{"type":"clear_thinking_20251015","keep":"all"}]}`
+			if next, ok := setJSONRawBytes(out, "context_management", []byte(cmDefault)); ok {
+				out = next
+				modified = true
+			}
+		}
+	}
+
 	if gjson.GetBytes(out, "tool_choice").Exists() {
 		if next, ok := deleteJSONPathBytes(out, "tool_choice"); ok {
 			out = next