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_test.go

@@ -0,0 +1,153 @@
+package models
+
+import (
+	"testing"
+)
+
+func TestExtractOpenAIUsage(t *testing.T) {
+	cases := []struct {
+		name   string
+		body   map[string]any
+		want   *TokenUsage
+		wantOK bool
+	}{
+		{
+			name: "standard openai",
+			body: map[string]any{
+				"usage": map[string]any{
+					"prompt_tokens":     float64(10),
+					"completion_tokens": float64(20),
+					"total_tokens":      float64(30),
+				},
+			},
+			want:   &TokenUsage{PromptTokens: 10, CompletionTokens: 20, TotalTokens: 30},
+			wantOK: true,
+		},
+		{
+			name: "total missing falls back to sum",
+			body: map[string]any{
+				"usage": map[string]any{
+					"prompt_tokens":     float64(10),
+					"completion_tokens": float64(20),
+				},
+			},
+			want:   &TokenUsage{PromptTokens: 10, CompletionTokens: 20, TotalTokens: 30},
+			wantOK: true,
+		},
+		{
+			name: "no usage key",
+			body: map[string]any{
+				"choices": []any{map[string]any{"message": map[string]any{"content": "hi"}}},
+			},
+			want:   nil,
+			wantOK: false,
+		},
+		{
+			name: "cohere style input_tokens/output_tokens",
+			body: map[string]any{
+				"usage": map[string]any{
+					"input_tokens":  float64(100),
+					"output_tokens": float64(50),
+				},
+			},
+			want:   &TokenUsage{PromptTokens: 100, CompletionTokens: 50, TotalTokens: 150},
+			wantOK: true,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, ok := extractOpenAIUsage(tc.body)
+			if ok != tc.wantOK {
+				t.Fatalf("extractOpenAIUsage() ok = %v, want %v", ok, tc.wantOK)
+			}
+			if !tc.wantOK {
+				return
+			}
+			if got.PromptTokens != tc.want.PromptTokens ||
+				got.CompletionTokens != tc.want.CompletionTokens ||
+				got.TotalTokens != tc.want.TotalTokens {
+				t.Fatalf("extractOpenAIUsage() = %+v, want %+v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestExtractOpenAIStreamUsage(t *testing.T) {
+	cases := []struct {
+		name   string
+		event  map[string]any
+		want   *TokenUsage
+		wantOK bool
+	}{
+		{
+			name: "standard streaming event",
+			event: map[string]any{
+				"usage": map[string]any{
+					"prompt_tokens":     float64(100),
+					"completion_tokens": float64(50),
+					"total_tokens":      float64(150),
+				},
+			},
+			want:   &TokenUsage{PromptTokens: 100, CompletionTokens: 50, TotalTokens: 150},
+			wantOK: true,
+		},
+		{
+			name: "null usage",
+			event: map[string]any{
+				"usage": nil,
+			},
+			want:   nil,
+			wantOK: false,
+		},
+		{
+			name: "no usage key",
+			event: map[string]any{
+				"choices": []any{map[string]any{"delta": map[string]any{"content": "hi"}}},
+			},
+			want:   nil,
+			wantOK: false,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, ok := extractOpenAIStreamUsage(tc.event)
+			if ok != tc.wantOK {
+				t.Fatalf("extractOpenAIStreamUsage() ok = %v, want %v", ok, tc.wantOK)
+			}
+			if !tc.wantOK {
+				return
+			}
+			if got.PromptTokens != tc.want.PromptTokens ||
+				got.CompletionTokens != tc.want.CompletionTokens ||
+				got.TotalTokens != tc.want.TotalTokens {
+				t.Fatalf("extractOpenAIStreamUsage() = %+v, want %+v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestExtractToken(t *testing.T) {
+	m := map[string]any{
+		"a": float64(1),
+		"b": int(2),
+		"c": int64(3),
+	}
+
+	if got := extractToken(m, "a"); got != 1 {
+		t.Fatalf("extractToken(a) = %d, want 1", got)
+	}
+	if got := extractToken(m, "b"); got != 2 {
+		t.Fatalf("extractToken(b) = %d, want 2", got)
+	}
+	if got := extractToken(m, "c"); got != 3 {
+		t.Fatalf("extractToken(c) = %d, want 3", got)
+	}
+	if got := extractToken(m, "missing"); got != 0 {
+		t.Fatalf("extractToken(missing) = %d, want 0", got)
+	}
+	if got := extractToken(m, "missing", "a"); got != 1 {
+		t.Fatalf("extractToken(missing, a) = %d, want 1", got)
+	}
+}