Fix: correct error handling for non-string keys in YAML validation

[kfess]

Description
This PR fixes the incorrect error messages that appear when YAML files contain non-string keys (numbers, null, boolean values) in the GitHub Actions VSCode extension.
I think the issue is caused by the YAML parser assuming all keys are strings without proper type conversion.

Changes Made

• Modified the YAML parser to properly convert all scalar values to strings before processing
• Replaced the type assertion as string with explicit string conversion using String()
• Added special handling for null values to convert them to empty strings, matching the GitHub Actions runner behavior
• Fixed JavaScript errors like i.indexOf is not a function that occurred with numeric keys
• Ensured boolean keys (true/false) are properly stringified

Fix #185

[Copilot]

Pull Request Overview

This PR fixes error handling during YAML validation by ensuring that non-string keys (null, boolean, numeric) are properly converted to strings.

• Replaces type assertion with explicit string conversion using String(), with special handling for null keys.
• Addresses JavaScript errors like "i.indexOf is not a function" by converting all scalar values to strings.

Comments suppressed due to low confidence (1)

workflow-parser/src/workflows/yaml-object-reader.ts:80

• Verify that using String() conversion properly handles numeric and boolean values as expected by the GitHub Actions runner behavior, and consider adding tests to cover these scenarios.

const key = scalarKey.value === null ? "" : String(scalarKey.value);