Auto-insert $schema when missing from DB file on startup

[Copilot]

When a DB file lacks a root-level $schema property, json-server now automatically inserts the README-documented default path, consistent with the existing id auto-fix behavior.

Changes

• src/service.ts: Added fixSchema(data) helper that sets $schema to ./node_modules/json-server/schema.json when absent; called in Service constructor alongside fixAllItemsIds
• src/service.test.ts: Extended constructor test to assert $schema insertion; added test asserting an existing $schema value is not overwritten

Behavior

// db.json before (missing $schema)
{ "posts": [{ "id": "1", "title": "a title" }] }

// db.data after Service construction
{
  "$schema": "./node_modules/json-server/schema.json",
  "posts": [{ "id": "1", "title": "a title" }]
}

Original prompt

Implement an auto-fix for missing $schema in JSON/JSON5 DB files, similar in spirit to existing auto-fix behavior for missing id values.

Repository: typicode/json-server

Goal

When json-server reads a DB file and the root object does not include $schema, it should automatically insert:

"$schema": "./node_modules/json-server/schema.json"

This exact path must match what is documented in the README usage example.

Requirements

1. Add logic in the same code path where DB normalization/auto-fix currently happens (where ids are auto-fixed) so that:
   • If root $schema is missing, it is added with value ./node_modules/json-server/schema.json.
   • If $schema already exists, leave it unchanged.
2. Preserve existing id auto-fix behavior; do not regress current functionality.
3. Ensure behavior applies to normal startup/write flow in a way consistent with existing auto-fixes.
4. Add/update tests to cover:
   • Missing $schema gets inserted.
   • Existing $schema is not overwritten.
   • Existing id auto-fix tests still pass (or are updated only as needed).
5. Run/update formatting and tests according to repository standards.

Notes

• Use the README-documented path exactly: ./node_modules/json-server/schema.json.
• Keep the change minimal and aligned with existing code style and architecture.
• If there is a dedicated helper for auto-fixes/default data normalization, extend it there rather than introducing parallel logic.

This pull request was created from Copilot chat.

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