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_body_order_test.go

@@ -41,12 +41,13 @@ func TestNormalizeClaudeOAuthRequestBody_PreservesTopLevelFieldOrder(t *testing.
 	resultStr := string(result)
 
 	require.Equal(t, claude.NormalizeModelID("claude-3-5-sonnet-latest"), modelID)
-	assertJSONTokenOrder(t, resultStr, `"alpha"`, `"model"`, `"system"`, `"messages"`, `"omega"`, `"tools"`, `"metadata"`)
-	require.NotContains(t, resultStr, `"temperature"`)
+	assertJSONTokenOrder(t, resultStr, `"alpha"`, `"model"`, `"temperature"`, `"system"`, `"messages"`, `"omega"`, `"tools"`, `"metadata"`, `"max_tokens"`)
+	require.Contains(t, resultStr, `"temperature":0.2`)
 	require.NotContains(t, resultStr, `"tool_choice"`)
 	require.Contains(t, resultStr, `"system":"`+claudeCodeSystemPrompt+`"`)
 	require.Contains(t, resultStr, `"tools":[]`)
 	require.Contains(t, resultStr, `"metadata":{"user_id":"user-1"}`)
+	require.Contains(t, resultStr, `"max_tokens":128000`)
 }
 
 func TestInjectClaudeCodePrompt_PreservesFieldOrder(t *testing.T) {