added use of constants for string literals to improve maintainability

[malik672]

chat gpt wrote this, was too lazy to think of something
Commit Description: Introduce constants for frequently used string literals

Motivation: This commit replaces direct string values with named constants at the beginning of the code. The constants represent commonly used string literals, such as keys in JSON maps ("id", "subscription", "result", "error"). The motivation behind this change is to enhance code maintainability, readability, and consistency.

Details:

• Constants like ID_KEY, SUBSCRIPTION_KEY, RESULT_KEY, and ERROR_KEY have been introduced at the beginning of the code.
• Instances where these string literals were used have been updated to use the corresponding constants.

Benefits:

1. Maintainability:

   • Changes to string literals, required by external API modifications or data format updates, can be made at a single point in the codebase.

2. Readability:

   • Named constants improve code readability by providing meaningful identifiers for commonly used string literals.

3. Consistency:

   • The use of constants enforces a consistent approach to referencing these string literals, minimizing the risk of typos or variations.

4. Refactoring and Collaboration:

   • Simplifies code refactoring and enhances collaboration by offering self-documenting elements that convey the purpose of each constant.

5. Ease of Maintenance:

   • Contributes to long-term maintainability by serving as documentation and organization aids in the evolving codebase.

Example: Before:

`match key {
    "id" => {
        // Handle "id" case
    }
    "subscription" => {
        // Handle "subscription" case
    }
  
}` 

After:

`const ID_KEY: &str = "id";
const SUBSCRIPTION_KEY: &str = "subscription";


match key {
    ID_KEY => {
        // Handle "id" case
    }
    SUBSCRIPTION_KEY => {
        // Handle "subscription" case
    }
  
}` 

This commit adheres to best practices, enhancing code quality and fostering a more maintainable and collaborative development environment.