Single entity REST endpoints (e.g., /feeds/{feed_id}, /routes/{route_key}) now return HTTP 404 when the entity doesn't exist, instead of HTTP 200 with empty results. Collection endpoints continue to return HTTP 200 with empty arrays.

Changes

Handler architecture

• Split request handlers into makeIndexHandler() (collections) and makeEntityHandler() (single entities)
• Entity handlers check for empty responses and return 404; index handlers return 200 with empty arrays
• Removed SingleEntity field from request structs—now determined by handler registration

Format support

• Extended 404 behavior to all response formats: JSON, GeoJSON, GeoJSONL, PNG
• PNG format now returns nil for empty results to enable 404 response

OpenAPI schema

• Added 404 response definitions to all entity endpoint specifications
• Fixed documentation typos in path templates (.format → .{format})

Test coverage

• Added assertions for 404 responses on missing entities (feeds, routes, stops, trips)
• Added assertions for 200 responses on empty collections
• Verified behavior across multiple formats (JSON, GeoJSON, PNG)

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