Enhance and refactor Gitleaks workflow with config validation

[hel-isa]

This pull request improves the Gitleaks integration in the .github/workflows/reusable-secrets.yml workflow by ensuring a config file is always present, adding stricter shell error handling, and making the Gitleaks execution more robust. The main changes are grouped below:

Gitleaks Configuration and Execution Enhancements:

• Added a step to check for the existence of the Gitleaks config file at the path specified by inputs.config_path. If the config file does not exist, a default configuration is created with sensible allowlist paths.
• Introduced set -euo pipefail at the start of relevant shell script steps to enforce stricter error checking and halt on errors or unset variables.
• Updated the Gitleaks run command to use multi-line formatting for better readability and maintainability.

Reporting Improvements:

• Minor formatting changes in the Python snippet that counts Gitleaks findings to improve readability.