Fix elixir 1.18 type warning

[Nitrino]

Fixes #238

PR Overview

This PR addresses an Elixir 1.18 type warning by updating the CI workflow configuration and adjusting the build steps in the workflow.

• Updated the Elixir version in the CI matrix from 1.17.2 to 1.18.2
• Added explicit compilation steps for the MIX_ENV=test environment before running tests for different decimal blends

Reviewed Changes

File	Description
.github/workflows/ci.yml	Updated Elixir version and inserted explicit compile steps before tests.

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

.github/workflows/ci.yml:27

• The explicit compilation step added to address the Elixir 1.18 type warning could benefit from an inline comment explaining its necessity for future maintainers.

      - run: MIX_ENV=test mix compile

.github/workflows/ci.yml:30

• [nitpick] The duplicate compile commands for different test blends might be consolidated if they serve an identical purpose, reducing redundancy in the workflow.

      - run: MIX_ENV=test mix compile