fix(skills): prevent empty strings overriding optional env vars in node scripts

[twishabansal]

Description

Fixes an issue where optional environment variables (e.g., ${CLOUD_SQL_POSTGRES_PASSWORD:}) were being passed as empty strings to the underlying toolbox process by the generated Node.js skill scripts. This disrupted fallback logic (like implicit IAM authentication).
The ConfigParser now tracks optional variables during YAML parsing. The generator uses this list to embed logic in the Node wrapper script to delete the environment variable from the spawned process if it evaluates to "".

Note: If a variable is marked as required (e.g., ${PROJECT_ID}) in one source, but optional (e.g., ${PROJECT_ID:}) in another within the same configuration file, the parser strictly enforces the required constraint, ensuring it is not mistakenly omitted when empty. Eg. in the cloud sql postgres source, CLOUD_SQL_POSTGRES_PROJECT is treated as required despite being optional for the admin source.

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🛠️ Fixes #<issue_number_goes_here>

[gemini-code-assist]

Code Review

This pull request introduces support for optional environment variables, ensuring they are omitted from the environment in generated Node.js scripts when empty. A critical issue was found in the script generator where duplicate declarations would cause a syntax error and break npx mode. Additionally, the configuration parser's duplicate check should be refactored to use the more idiomatic slices.Contains function.

[twishabansal]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to track and handle optional environment variables within the configuration parser and generated wrapper scripts. The ConfigParser now distinguishes between required and optional variables based on the presence of default values in the configuration. Additionally, the generated Node.js wrapper scripts are updated to automatically omit optional environment variables if they are set to empty strings. I have no feedback to provide as there were no review comments to assess.

[twishabansal]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces tracking for optional and required environment variables within the ConfigParser. This metadata is utilized during the generation of Node.js wrapper scripts to ensure that optional variables are omitted from the environment if they are empty. The review feedback suggests unexporting the RequiredEnvVars field to improve encapsulation and refactoring the environment variable tracking logic to use more idiomatic functions from the slices package.

[github-actions]

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