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_anthropic_apikey_passthrough_test.go

@@ -762,8 +762,14 @@ func TestGatewayService_AnthropicOAuth_ForwardPreservesBillingHeaderSystemBlock(
 			system := gjson.GetBytes(upstream.lastBody, "system")
 			require.True(t, system.Exists())
 			require.True(t, system.IsArray(), "system should be an array")
-			require.Equal(t, claudeCodeSystemPrompt, system.Array()[0].Get("text").String())
-			require.Equal(t, "ephemeral", system.Array()[0].Get("cache_control.type").String())
+			arr := system.Array()
+			require.Len(t, arr, 2, "system array should have billing block + cc prompt block")
+
+			require.Contains(t, arr[0].Get("text").String(), "x-anthropic-billing-header:")
+			require.Contains(t, arr[0].Get("text").String(), "cc_version=")
+
+			require.Equal(t, claudeCodeSystemPrompt, arr[1].Get("text").String())
+			require.Equal(t, "ephemeral", arr[1].Get("cache_control.type").String())
 
 			// 原始 system prompt 应迁移至 messages 中
 			messages := gjson.GetBytes(upstream.lastBody, "messages")