feat: prefer OpenAI accounts by remaining quota

[MiaoZang]

Summary

• add quota_remaining account selection mode for same-priority account selection
• add optional quota balance snapshot caching for quota-based ranking
• add OpenAI OAuth quota strategy helpers and reset-window-aware remaining quota calculation
• add optional sticky session release when a same-priority OpenAI account has a much higher remaining quota
• include small Windows compile fixes needed for local Go test runs on Windows

Tests

• GOTMPDIR=T:\C++save\GPT\sub2api部署\tmp-go go test -tags unit ./internal/config ./internal/service -run 'TestLoad.*Scheduling|TestEffectiveRemainingQuota|TestGatewayService.*Quota|TestGatewayService_ShouldReleaseOpenAIStickySessionForQuotaGap|TestGatewayService_ShouldNotReleaseOpenAIStickySessionForQuotaGapAcrossPriority|TestOpenAIQuotaStrategySchedulable|TestParseGatewayRequest_MaxTokensBoundary' -count=1 -v
• git diff --cached --check