feat: add dependentSchemas keyword handler

[vanshaj2023]

Summary

Implements the dependentSchemas keyword handler in processAST.ts.

The keyword was previously commented out, causing it to fall back to fallbackHandler and display a "not implemented" warning node. This PR adds a proper handler that recursively creates child nodes for each dependent subschema.

The handler was designed by tracing Hyperjump's source (lib/keywords/dependentSchemas.js) — dependentSchemas compiles via asyncCollectArray, producing [string, string][] tuples of [propertyName, schemaUri]. The property name is used as childId (not a numeric index like patternProperties) so source handles are meaningful, consistent with the properties handler convention.

What kind of change does this PR introduce

New feature — adds a missing keyword handler

Issue Number

Closes #

Screenshots/Video

Schema used for testing:

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "credit_card": { "type": "string" }
  },
  "dependentSchemas": {
    "credit_card": {
      "properties": { "billing_address": { "type": "string" } },
      "required": ["billing_address"]
    }
  }
}

Does this PR introduce a breaking change?

No

If relevant, did you update the documentation?

No

[adityayadav-dev]

Hey @vanshaj2023, since this also falls under the GSoC qualification tasks, I believe it's intended to be part of the proposal rather than a PR—as mentioned in the task description. Opening a PR at this stage might create some unnecessary overhead for the maintainers. Thanks!