fix: add overflow/underflow protection to counter example contracts

[emmanuelist]

Description

Fixes compiler warnings about "use of potentially unchecked data" in the counter example contracts by adding proper overflow and underflow protection.

Changes

• Add pre-operation validation to prevent arithmetic overflow in increment function
• Add pre-operation validation to prevent underflow in decrement function
• Return error u1 on overflow attempt, u2 on underflow attempt
• Add comprehensive tests to verify overflow/underflow protection

Testing

• All existing tests pass
• Added new test for underflow protection
• Added new test for overflow protection (u128 max value)
• No compiler warnings (clarinet check passes cleanly)

Impact

• Provides safer example code for developers learning Clarity
• Demonstrates best practices for arithmetic operations
• Eliminates all warnings from the counter example contracts

[hugo-stacks]

Since this is just an example, I'd like to keep it simple. What do you think?