Raise test coverage above 95%

[renkun-ken]

Summary

• add focused tests across language-feature handlers, providers, server lifecycle, workspace behavior, protocol types, and native utilities
• cover semantic-token cache/fallback paths, missing documents, callback cleanup, and error handling
• fix S3 method-definition lookup and empty help-file handling exposed by the new tests

Testing

• 2,051 assertions passed
• 0 failures or warnings
• 1 existing empty-test skip
• package coverage: 96.11%