fix(MergedSpecBuilder): merge path items by HTTP method instead of overwriting on path collision

[Picazsoo]

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Summary by cubic

Fixes OpenAPI merging to combine colliding paths by HTTP method and keep both operations. Builds a full merged spec with deduped components/servers and preserved vendor extensions.

• Bug Fixes

  • Merge path items by method; warn on duplicate path+method and keep the first.
  • Merge components by name; dedupe identical ones; warn on conflicts.
  • Preserve x- vendor extensions on paths, operations, and schemas.
  • Deduplicate server URLs; add http://localhost:8080 if none exist.

• Refactors

  • Build and serialize a full OpenAPI model via swagger Json/Yaml mappers.
  • Only process .yaml, .yml, .json; ignore invalid files; deterministic ordering; choose JSON output if the first spec is JSON.
  • Add tests for path collisions, component merging, vendor extensions, identical duplicates, non-spec files, and conflict warnings; update petstore-v3.1.yaml operationIds to avoid test collisions.

^{Written for commit 818f196. Summary will update on new commits.}