Refactor/openapi dedupe errors

[oericdacosta]

refactor: deduplicate parameters and standardize error responses

Description

This PR refactors the Shopping Service OpenAPI specification source/services/shopping/openapi.json to improve maintainability and strictly follow the DRY principle.

Key Changes:

1. Deduplicated Header Parameters: Moved 9 common header parameters (e.g., Authorization, X-API-Key, Request-Id) from individual operations to the path level. This significantly reduces redundancy across endpoints like /checkout-sessions and /carts.
2. Standardized Error Responses: Introduced a reusable error_response schema and added standard HTTP error responses (400, 401, 404, 500) to all operations, referencing shared definitions in components/responses.
3. Synced with Upstream: Aligned with the latest project structure by removing legacy spec/ artifacts, ensuring a clean merge with main

These changes make the OpenAPI spec more concise, easier to read, and ensure consistent error handling documentation across the API.

Type of change

Please delete options that are not relevant.

• Refactor (non-breaking change which improves code quality)

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Is this a Breaking Change or Removal?

If you checked "Breaking change" above, or if you are removing any schema
files or fields:

• I have added ! to my PR title (e.g., feat!: remove field).
• I have added justification below.

## Breaking Changes / Removal Justification
N/A - This is a purely structural refactor of the source OpenAPI file.

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules