Escape JSON special characters in Serialize.tryToJSONString

[janezpodhostnik]

Closes: #213

Description

Adds an escapeJSONString helper to Serialize.cdc that escapes \, ", and control characters (U+0000–U+001F) per RFC 8259 §7, and applies it in the String, String?, and Character cases of tryToJSONString. Dictionary keys in dictToJSONString are serialized through the same String case, so they are covered as well.

Escaping is a single pass over UTF-8 bytes with a fast path that returns the input unchanged when nothing needs escaping; multi-byte UTF-8 sequences pass through untouched.

Includes unit tests for all escape classes (quotes, backslash, \n/\r/\t, \b/\f, \u00XX control chars, multi-byte passthrough, dict keys) and a regression test serializing an NFT with quotes, a newline, and a tab in its name/description to a parseable JSON data URI.

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For contributor use:

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• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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