Support undefined, number in JSON module

[mario-campos]

Summary

This pull request introduces several additions to the JSON schema-validation utilities:

• Renamed the optional validator to optionalOrNull to clarify that schema fields may be optional and accept null as a valid value, in addition to undefined.
• Redefined the optional validator so that it only accepts undefined (not null), making its behavior more precise and distinct from optionalOrNull.
• Added new built-in validators for number.
• Added an isNumber type guard function for type checking.

This PR is a prerequisite for #3950.

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• Confirm this change is backwards compatible with existing workflows.
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[Copilot]

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Pull request overview

This PR extends the internal JSON schema-validation helpers to better distinguish “optional” vs “optional-or-null” fields, and to add first-class validators for number and object. It also updates start-proxy configuration schemas to use the new optionalOrNull semantics and adjusts unit tests accordingly.

Changes:

• Introduces isNumber, plus json.number and json.object validators.
• Renames the old “optional (undefined or null)” behavior to optionalOrNull and redefines optional to mean “undefined-only”.
• Updates start-proxy schemas and unit tests to reflect the new optional semantics.

Show a summary per file

File	Description
src/start-proxy/types.ts	Switches relevant schema fields from optional to optionalOrNull for backwards-compatible null acceptance.
src/json/index.ts	Adds number/object validators and splits optional handling into optionalOrNull vs optional.
src/json/index.test.ts	Updates/expands tests to cover the new optional semantics (optionalOrNull vs optional).
lib/entry-points.js	Generated output (content excluded from review per policy).

Review details

Files excluded by content exclusion policy (1)

• lib/entry-points.js

• Files reviewed: 3/4 changed files
• Comments generated: 3
• Review effort level: Low