Add draft-06 contains support

[Adityakumar37]

Hi @jdesrosiers

I made the required updates for draft-06/07 contains support:

Registered only the draft-06 keyword URI:
https://json-schema.org/keyword/draft-06/contains
(since draft-06 and draft-07 share the same keyword URI)

Added a draft-specific normalization handler at:
src/normalization-handlers/draft-06/contains.js
which implements the simpler draft-06 semantics (at least one matching item, no minContains/maxContains).

Updated the shared error handler (src/error-handlers/contains.js) to treat
draft-06/contains as a special case:

• skips minContains/maxContains processing
• always reports the default range { minContains: 1 }

All tests are passing locally, and lint/type-check are clean.

Please let me know if anything else should be adjusted.