fix: Add XYK pool existence check before conversion (#1027)

[Arsalaan-Alam]

Description

Added a check to verify if the XYK pool exists before performing conversion calculations in calculate_in_given_out. This prevents arithmetic errors and improves error handling with descriptive messages.

Related Issue

Fixes: #1027

Motivation and Context

This change ensures transactions do not fail due to missing XYK liquidity pools. Instead of encountering an arithmetic error, users will receive a clear DispatchError message indicating that the pool does not exist.

##How Has This Been Tested?

• Built the runtime successfully using cargo build
• Ran unit tests with cargo test --all
• Verified transactions involving non-existent XYK pools return a proper error instead of failing with ArithmeticError::Overflow.

Checklist:

• I have updated the documentation if necessary.
• I have added tests to cover my changes, including regression tests for missing XYK pools.
• This is a breaking change.

[dmoka]

Nice, please also add an integration test to verify the change and document this behaviour.

And also don't forget to bump runtime version.

[Arsalaan-Alam]

on it