fix: support lake SQLAlchemy URLs

[sdojjy]

Summary

• register the lake SQLAlchemy dialect and preserve the caller URL scheme
• update README connection examples to use lake:// and repair example imports
• correct the README package content type and add regression coverage

Validation

• connected to the supplied staging Lake endpoint through create_engine("lake://...") and ran SELECT 1 plus SELECT current_database()
• verified lake:// and legacy tidbcloudlake:// URL argument handling
• built the wheel and verified its dialect entry point
• git diff --check

[pingcap-cla-assistant]

All committers have signed the CLA.