fix: align byteplus viking kb tos region

[xgtcode]

Summary

Fix BytePlus Viking knowledge base TOS region alignment.

When creating or writing to a Viking knowledge base on BytePlus, the Viking region is normalized to cn-hongkong, but the default TOS config previously stayed on the BytePlus global default ap-southeast-1. This could make document upload and bucket creation/checking use different TOS regions.

This change:

• Aligns the default TOS region/endpoint to cn-hongkong for BytePlus Viking knowledge base usage.
• Preserves explicit TOS configuration from tos_config, DATABASE_TOS_REGION, or DATABASE_TOS_ENDPOINT.
• Ensures TOSConfig.bucket creates/checks the bucket with region=self.region, so bucket setup and later upload use the same TOS region.

Tests

• uv run pytest tests/test_vikingdb_knowledge_backend.py tests/test_ve_tos.py
• uv run pre-commit run --files veadk/configs/database_configs.py veadk/knowledgebase/backends/vikingdb_knowledge_backend.py tests/test_vikingdb_knowledge_backend.py