feat(go-models): unify OpenAI-compatible token usage extraction (#17634)

## Summary
Relate to #17284 .

- Add `usage_parser.go`: `extractOpenAIUsage` /
`extractOpenAIStreamUsage`
  with multi-field fallback (`prompt_tokens`/`input_tokens`,
  `completion_tokens`/`output_tokens`).
- Add `parser_config.go`: `ParserConfig` struct and
`OpenAIParserConfig`.
- Add `response_handler.go`: `HandleNonStreamingResponse` /
  `HandleStreamingResponse` as single entry points that extract usage,
  populate `chatConfig.UsageResult`, and emit a `StreamUsage` log.
- Extend `base_model.go` with `doRequest` / `doStreamRequest`.
- Migrate `deepseek` driver to shared handlers, cutting ~100 lines.
- Add `usage_parser_test.go`.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Haruko386 <tryeverypossible@163.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

internal/entity/models/usage_parser.go

@@ -0,0 +1,77 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package models
+
+import "encoding/json"
+
+// extractOpenAIUsage extracts token usage from an OpenAI-compatible
+// non-streaming response body. Returns (nil, false) when the response
+// carries no usable usage block.
+func extractOpenAIUsage(body map[string]any) (*TokenUsage, bool) {
+	rawUsage, ok := body["usage"].(map[string]any)
+	if !ok {
+		return nil, false
+	}
+
+	return tokenUsageFromRaw(rawUsage), true
+}
+
+// extractOpenAIStreamUsage extracts token usage from one OpenAI-compatible
+// streaming event. A missing or null usage field is not an error.
+func extractOpenAIStreamUsage(event map[string]any) (*TokenUsage, bool) {
+	rawUsage, ok := event["usage"].(map[string]any)
+	if !ok || rawUsage == nil {
+		return nil, false
+	}
+
+	return tokenUsageFromRaw(rawUsage), true
+}
+
+func tokenUsageFromRaw(rawUsage map[string]any) *TokenUsage {
+	usage := &TokenUsage{}
+	usage.PromptTokens = extractToken(rawUsage, "prompt_tokens", "input_tokens")
+	usage.CompletionTokens = extractToken(rawUsage, "completion_tokens", "output_tokens")
+	usage.TotalTokens = extractToken(rawUsage, "total_tokens")
+
+	if usage.TotalTokens == 0 {
+		usage.TotalTokens = usage.PromptTokens + usage.CompletionTokens
+	}
+
+	return usage
+}
+
+// extractToken reads a numeric field from a map, trying each key in order.
+// Returns 0 when no key is present or the value is not a number.
+func extractToken(m map[string]any, keys ...string) int {
+	for _, k := range keys {
+		if v, ok := m[k]; ok {
+			switch val := v.(type) {
+			case float64:
+				return int(val)
+			case int:
+				return val
+			case int64:
+				return int(val)
+			case json.Number:
+				if n, err := val.Int64(); err == nil {
+					return int(n)
+				}
+			}
+		}
+	}
+	return 0
+}