ajv 8.20.0: Strip undeclared object properties in Ajv by pairing removeAdditional true with additionalProperties false schemas

# Ajv removeAdditional Property Stripping Behavior

## Prior Knowledge Search
Querying existing records for `ajv` returned three distinct published samples:
- `sha256:d0612b722f729d61a5720643776ca58d07362ba8ff7616968d4d9b67559b1d84` covering async validation error rejection with `ValidationError`.
- `sha256:e38b0edd91808843f1d8b3083043259db544de2270eefd310cf866ce39d955fe` covering schema default value population via `useDefaults`.
- `sha256:6c0869519510ebd76ed0393a2bdecf6028623f29ba188f44368ecabd4e7668a9` covering scalar to array type coercion via `coerceTypes`.

This sample targets an orthogonal and critical data mutation behavior: how `removeAdditional` interacts with schema-level `additionalProperties` definitions.

## Pinned Release
- Package: `ajv`
- Version: `8.20.0` (exact match in `package.json` and `package-lock.json`)

## Observed Behaviour
When initializing `Ajv` with `removeAdditional: true`:
1. Undeclared properties are stripped from input data objects in-place during validation only if the schema explicitly sets `additionalProperties: false`. Validation returns `true` and the mutated object contains only the allowed properties.
2. If `additionalProperties` is omitted (or set to `true`), `removeAdditional: true` does not remove undeclared properties. The input object retains all unexpected keys while validation returns `true`.
3. Setting `removeAdditional: "all"` strips undeclared properties even when `additionalProperties: false` is not specified in the schema.
4. Setting `removeAdditional: "failing"` validates extra properties against the subschema defined in `additionalProperties` (such as `{ type: "number" }`), removing only properties that fail validation and retaining those that pass.
5. In standard `Ajv` (without `removeAdditional`), providing unexpected fields to a schema with `additionalProperties: false` causes validation to fail with `valid === false` and an `additionalProperties` error.

## Failure Mode
The naive belief is that enabling `removeAdditional: true` on an `Ajv` instance acts as a global sanitizer that strips undeclared fields from all objects validated against structured schemas.

This expectation fails silently with a green build: validation returns `true`, yet unexpected properties remain untouched in the data object unless `additionalProperties: false` was explicitly written on that specific object schema. In security-sensitive contexts (such as sanitizing untrusted request payloads before database insertion), this discrepancy can lead to silent data pollution.

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import assert from 'node:assert';
import Ajv from 'ajv';

// 1. removeAdditional: true ONLY strips properties when additionalProperties: false is set
{
  const ajv = new Ajv({ removeAdditional: true });

  const schemaWithoutFlag = {
    type: 'object',
    properties: {
      id: { type: 'integer' },
      name: { type: 'string' }
    }
  };
  const validateWithoutFlag = ajv.compile(schemaWithoutFlag);
  const dataKept = { id: 1, name: 'Alice', extraField: 'should_stay' };
  const validKept = validateWithoutFlag(dataKept);

  assert.strictEqual(validKept, true);
  // Undeclared property is preserved because additionalProperties: false was omitted
  assert.strictEqual('extraField' in dataKept, true);
  assert.deepStrictEqual(dataKept, { id: 1, name: 'Alice', extraField: 'should_stay' });

  const schemaWithFlag = {
    type: 'object',
    properties: {
      id: { type: 'integer' },
      name: { type: 'string' }
    },
    additionalProperties: false
  };
  const validateWithFlag = ajv.compile(schemaWithFlag);
  const dataStripped = { id: 1, name: 'Alice', extraField: 'should_be_removed' };
  const validStripped = validateWithFlag(dataStripped);

  assert.strictEqual(validStripped, true);
  // Undeclared property is stripped in-place
  assert.strictEqual('extraField' in dataStripped, false);
  assert.deepStrictEqual(dataStripped, { id: 1, name: 'Alice' });
}

// 2. Default Ajv without removeAdditional fails validation when additionalProperties is false
{
  const ajvDefault = new Ajv();
  const schemaStrict = {
    type: 'object',
    properties: {
      id: { type: 'integer' }
    },
    additionalProperties: false
  };
  const validateStrict = ajvDefault.compile(schemaStrict);
  const data = { id: 10, unexpected: 'value' };
  const isValid = validateStrict(data);

  assert.strictEqual(isValid, false);
  assert.strictEqual(validateStrict.errors[0].keyword, 'additionalProperties');
  assert.strictEqual('unexpected' in data, true);
}

// 3. removeAdditional: 'all' removes undeclared properties even without additionalProperties: false
{
  const ajvAll = new Ajv({ removeAdditional: 'all' });
  const schema = {
    type: 'object',
    properties: {
      id: { type: 'integer' }
    }
  };
  const validateAll = ajvAll.compile(schema);
  const data = { id: 2, extraField: 'removed_by_all' };
  const isValid = validateAll(data);

  assert.strictEqual(isValid, true);
  assert.strictEqual('extraField' in data, false);
  assert.deepStrictEqual(data, { id: 2 });
}

// 4. removeAdditional: 'failing' strips only properties that fail additionalProperties schema
{
  const ajvFailing = new Ajv({ removeAdditional: 'failing' });
  const schema = {
    type: 'object',
    properties: {
      id: { type: 'integer' }
    },
    additionalProperties: { type: 'number' }
  };
  const validateFailing = ajvFailing.compile(schema);
  const data = { id: 3, allowedScore: 95, disallowedTag: 'invalid_string' };
  const isValid = validateFailing(data);

  assert.strictEqual(isValid, true);
  assert.strictEqual('disallowedTag' in data, false);
  assert.strictEqual(data.allowedScore, 95);
  assert.deepStrictEqual(data, { id: 3, allowedScore: 95 });
}