fix: escape JSON Pointer tokens in toJSONSchema $ref (RFC 6901)

[godfengliang]

Fixes #6027

Problem

When a schema id contains / or ~ characters, the $ref URI fragment was not being escaped per RFC 6901 (JSON Pointer):

• ~ should become ~0
• / should become ~1

For example, with id Shared/User~, the $ref was:
#/$defs/Shared/User~ (incorrect)

Now correctly produces:
#/$defs/Shared~1User~0

The $defs object key remains unescaped (as it's a plain JSON object key, not a JSON Pointer).

Fix

Apply split("~").join("~0").split("/").join("~1") to the defId in both $ref construction paths in extractDefs:

1. The shared schema path (line ~263)
2. The self-contained schema path (line ~274)

Testing

Verified with three test cases:

• id: "Shared/User~" -> $ref: "#/$defs/Shared~1User~0" ✓
• id: "MySchema" -> $ref: "#/$defs/MySchema" ✓ (no change for normal ids)
• id: "My~Schema" -> $ref: "#/$defs/My~0Schema" ✓

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