enhance(build): validate built data.json against schemas

[caugner]

Summary

1. Validates the built data.json against the schemas.
2. Removes the maxProperties requirement for the browsers object from the browsers schema.

Test results and supporting details

This revealed two minor issues:

1. The browsers schema was requiring a maximum of one browser defined in browsers. This requirement only applies to the individual browser/*.json files, not the built data.json, so this requirement was removed.
2. The __meta object in the built data.json is not currently covered by the schema, so we cannot validate it.

Related issues

Fixes #27564.

[github-actions]

Tip: Review these changes grouped by change (recommended for most PRs), or grouped by feature (for large PRs).

[caugner]

Note

If validation fails, the validation errors are logged via console.error, but we don't bail out, to avoid disruption for contributors in the unlikely event that this ever happens on main. In either case, these errors won't get unnoticed, because they will appear on every npm install (via the prepare script).

[ddbeck]

• The __meta object in the built data.json is not currently covered by the schema, so we cannot validate it.

FWIW, I raised this as a concern in #17574 and got shot down on doing anything about it.

[caugner]

FWIW, I raised this as a concern in #17574 and got shot down on doing anything about it.

FWIW, here's the meta schema (isolated, like the other two):

{
  "$schema": "http://json-schema.org/schema",

  "definitions": {
    "meta_block": {
      "type": "object",
      "properties": {
        "timestamp": {
          "type": "string",
          "format": "date-time"
        },
        "version": {
          "type": "string"
        }
      },
      "required": ["timestamp", "version"],
      "additionalProperties": false
    }
  },

  "properties": {
    "__meta": {
      "$ref": "#/definitions/meta_block"
    }
  },

  "title": "MetaData",
  "type": "object",
  "required": ["__meta"],
  "additionalProperties": false,
  "maxProperties": 1,
  "minProperties": 1
}

Unfortunately, ajv gives me an error when validating the data:

TYPE must be string

  2 |   "__meta": {
  3 |     "version": "7.1.0",
> 4 |     "timestamp": "2025-08-28T14:30:46.924Z"
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^ 👈🏽  type must be string
  5 |   }
  6 | }

That error doesn't make sense, but given this is not a priority, I would declare the scope of this PR to validate the built data against the existing schemas.