qs 6.15.3: Preserve conflicting scalar and object query values in an array via strictMerge introduced in qs 6.15.0 instead of mutating scalars into boolean object keys

# Notes

## Search Result
`search_known_solution` returned existing sample `sha256:8e9326c0c41845367e7ad94c06a66850cf8323754e7cb974afd73f6b947b2fde` ("Parse and build nested query strings that URLSearchParams cannot express"), so this contract addresses a distinct semantic boundary trap: `strictMerge` handling of scalar/object collisions introduced in `qs@6.15.0`.

## Naive Model Expectation
A model expects parsing conflicting parameters like `user[name]=alice&user=bob` to either overwrite `user` with `'bob'`, throw an error, or symmetrically construct an array regardless of version.

## Version Failure Mode
In versions prior to 6.15.0 (or with `strictMerge: false`), `qs.parse('user[name]=alice&user=bob')` silently mutates the scalar `'bob'` into an object property `{ user: { name: 'alice', bob: true } }`, causing silent data loss where builds pass green but downstream consumers receive `true` instead of `'bob'`.

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{
  "name": "csx-sample-qs-strict-merge",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "qs": "6.15.3"
  }
}

import qs from 'qs';

/**
 * Parse query string using qs.
 * In qs >= 6.15.0, strictMerge defaults to true, preventing conflicting
 * scalar and object keys from mutating the scalar into a boolean object property.
 */
export function parseQuery(queryString, options = {}) {
  return qs.parse(queryString, options);
}

export function stringifyQuery(obj, options = {}) {
  return qs.stringify(obj, options);
}

import { strict as assert } from 'node:assert';
import { parseQuery } from '../src/index.mjs';

// 1. In qs >= 6.15.0, strictMerge defaults to true.
// Conflicting nested object and scalar keys are wrapped into a polymorphic array.
const parsed = parseQuery('user[name]=alice&user=bob');
assert.equal(
  Array.isArray(parsed.user),
  true,
  'qs >= 6.15.0 must wrap conflicting nested object and scalar keys in an array by default'
);
assert.deepEqual(
  parsed.user,
  [{ name: 'alice' }, 'bob'],
  'scalar value bob must be preserved as an array element rather than mutated into an object key'
);
assert.equal(
  parsed.user.bob,
  undefined,
  'bob must not be a boolean property key on the user object'
);

// 2. Order symmetry in qs >= 6.15.0:
// Whether scalar precedes object or follows it, both are collected into an array.
const parsedScalarFirst = parseQuery('user=bob&user[name]=alice');
assert.equal(
  Array.isArray(parsedScalarFirst.user),
  true,
  'scalar-first collision must also produce an array'
);
assert.deepEqual(
  parsedScalarFirst.user,
  ['bob', { name: 'alice' }],
  'scalar bob precedes nested object when it appeared first in query'
);

// 3. Legacy pre-6.15.0 behavior via explicit strictMerge: false
// Demonstrates the silent corruption trap: scalar "bob" became key "bob": true
const legacy = parseQuery('user[name]=alice&user=bob', { strictMerge: false });
assert.equal(
  Array.isArray(legacy.user),
  false,
  'pre-6.15.0 legacy merge returned a mutated object instead of an array'
);
assert.equal(
  legacy.user.name,
  'alice',
  'original object property is preserved'
);
assert.equal(
  legacy.user.bob,
  true,
  'pre-6.15.0 silently converted the scalar value into a boolean property key'
);

// 4. Bracket vs scalar duplicate handling (fixed in 6.15.0 alongside strictMerge)
// In 6.15.0+, duplicates: 'last' only deduplicates unbracketed keys; explicit bracket arrays keep all elements.
const bracketDuplicates = parseQuery('user[]=alice&user[]=bob', { duplicates: 'last' });
assert.deepEqual(
  bracketDuplicates.user,
  ['alice', 'bob'],
  'explicit bracket array notation preserves all entries under duplicates: last in 6.15.0+'
);

const scalarDuplicates = parseQuery('user=alice&user=bob', { duplicates: 'last' });
assert.equal(
  scalarDuplicates.user,
  'bob',
  'unbracketed scalar keys respect duplicates: last'
);

console.log('CONTRACT PASS: qs 6.15.0+ preserves conflicting scalar/object values via strictMerge without mutating scalars into boolean object keys');