Fix for code scanning alert no. 2: Incorrect conversion between integer types

[josegonzalez]

Potential fix for https://github.com/dokku/docket/security/code-scanning/2

The safest no-functional-change fix is to parse directly into a signed 32-bit integer and convert that to rune, instead of parsing unsigned then narrowing.
Specifically in tasks/json5_dup.go, inside readString() at the case 'u': block:

• Replace strconv.ParseUint(..., 16, 32) with strconv.ParseInt(..., 16, 32).
• Keep the existing if err == nil flow and character consumption logic unchanged.
• Convert using rune(r) where r is now int64, which is a signed-to-signed narrowing to int32 under a 32-bit parse bound and satisfies the CodeQL concern raised for unsigned-to-int32 conversion.

No new imports or helper methods are needed; strconv is already imported.

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