Add test contract that imports wasmbindings

[jhelison]

Is your feature request related to a problem? Please describe.
No example contract actually imports and uses these bindings. That’s the most crucial teaching tool.

Describe the solution you'd like
Create a sample CosmWasm contract in examples/sample_contract/ that:

• Uses wasmbindings
• Performs a query or binds a struct
• Compiles to wasm

Describe alternatives you've considered
Isolated usage in lib.rs—but full contract context is more realistic.

Additional context
This can also serve for integration tests later.