Fix environment extension activation with scoped settings - #26117

Summary

• align useEnvExtension() with the Python Environments extension's window-wide explicit-disable rules
• fall back to the legacy environment path when any configuration scope explicitly disables Python Environments
• add regression coverage for conflicting global/workspace values and the enabled path
• isolate the memoized activation decision between tests

Fixes #26068

Validation

• ./node_modules/.bin/tsc -p ./
• ESLint on the changed source and test files
• focused affected suites: 32 passing, 3 pending
• full unit suite: 5,170 passing, 36 pending, with 2 unrelated existing Posix Known Path Locator failures