added changesets

.changeset/odd-buckets-compare.md

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+---
+'@backstage/backend-common': patch
+---
+
+Set explicit content-type in error handler responses.

.changeset/strange-cobras-unite.md

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+---
+'@backstage/plugin-catalog-backend': patch
+---
+
+Refactored route response handling to use more explicit types and throw errors.