Add error response format integration test

[luisvlz0]

DESCRIPTION

Adds an integration test for OpenCost API error response formatting. This test verifies that invalid API requests return a structured error response instead of an unexpected or malformed response body.

CHANGES

• Adds a new error_response_format integration test under test/integration/api
• Adds a test.bats file to run the test
• Validates that API error responses include the expected format and status behavior

MOTIVATION

This helps ensure API clients receive consistent and predictable error responses when requests are invalid. It also provides regression coverage so future API changes do not accidentally break error response formatting.

TESTING

This test can be run from the new test directory using the included Bats test file.

Signed-off-by: luisvaldez2h@gmail.com

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