fix: preserve Nacos metadata and clamp weights

[spduan78]

Summary

• clamp rounded Nacos instance weights so healthy positive weights never become 0
• apply the same weight normalization to Nacos v1 and v2 watchers
• preserve v1 instance metadata labels when protocol is absent

Tests

• GOCACHE=/private/tmp/higress-go-cache go test ./registry/nacos -run 'Test_generateServiceEntry_(Weight|WeightFieldSet)' -count=1
• GOCACHE=/private/tmp/higress-go-cache go test ./registry/nacos/v2 -run 'Test_generateServiceEntry_(Weight|WeightFieldSet)' -count=1
• GOCACHE=/private/tmp/higress-go-cache go test ./registry/nacos ./registry/nacos/v2

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