Bump cw version

[Ninjatosba]

CW v3 upgrade + test cleanup

Upgraded to CosmWasm v3 and completely refactored the test suite.

What I did

• Bumped CW to v3
• Broke up the giant src/tests.rs file into separate test files
• Removed old migration code that was cluttering things
• Fixed all the linter warnings

Test coverage

Got it up to 94% with grcov. Split tests into logical groups:

• create_stream, subscribe, withdraw
• killswitch, threshold, exit
• operator, protocol_admin, price_feed
• etc.

The test structure is way cleaner now and easier to maintain. Each feature has its own test file instead of one massive file.