Add schema checker to json module

[mbg]

This PR cherry-picks some commits from #3994 which extend the json module with a schema checker that returns information about which keys are unknown or invalid in the provided value according to the given schema.

This also includes new object and array validators that are parametrised over schemas and other validators, respectively.

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• Confirm this change is backwards compatible with existing workflows.
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Pull request overview

Extends the JSON validation module with composable validators and detailed schema-checking results, supporting future configuration merging work.

Changes:

• Adds array and object validators.
• Adds unknown/invalid key reporting and fail-fast validation.
• Adds unit tests for nested schemas and diagnostics.

Show a summary per file

File	Description
src/json/index.ts	Implements validators and schema checking.
src/json/index.test.ts	Tests arrays, objects, and key reporting.
lib/entry-points.js	Generated bundle; excluded from review.

Review details

Files excluded by content exclusion policy (1)

• lib/entry-points.js

• Files reviewed: 2/3 changed files
• Comments generated: 3
• Review effort level: Medium