Multi-spec merge fails on equivalent duplicate schemas

## Summary Generating from multiple `openApiPaths` fails when Refitter encounters schema conflicts like in the e-conomic OpenAPI specs (`economic-products.json` and `economic-webhooks.json`) Both OpenAPI files define shared schemas such as `Error` and `ProblemDetails`. Those schemas are equivalent, but the merge logic treats them as hard conflicts and stops generation. > The OpenAPI files should **not** be modified to work around this. ## Repro 1. Use the existing `test\multiple-sources.refitter` file: ```json { "openApiPaths": [ "OpenAPI/v3.0/economic-products.json", "OpenAPI/v3.0/economic-webhooks.json" ], "namespace": "Economic", "outputFolder": "GeneratedCode" } ``` 2. Run Refitter against that file. ## Expected behavior Refitter should merge the two specs, accept equivalent duplicate schemas, and generate non-empty, buildable code. ## Actual behavior Generation fails during multi-spec merge with a duplicate schema error, even though the duplicated schemas are structurally the same. ## Root cause The merge logic failed fast on duplicate schema names unless the existing and incoming values compared equal in a narrow way. That was too strict for equivalent schemas loaded from separate documents, especially when schemas referenced other shared schemas such as `ProblemDetails -> Error`. ## Fix summary - Compare duplicate schemas structurally instead of treating every same-name duplicate as a conflict. - Keep fail-fast behavior for real schema differences. - Add regression coverage for equivalent duplicate schema handling, including the real e-conomic multi-spec scenario.