fix(gnovm): respect Unicode range in 64-bit integer-to-string conversions

[omarsy]

Description

Per the Go spec (Conversions to and from a string type), converting an integer to a string yields the UTF-8 of the code point, and "values outside the range of valid Unicode code points are converted to �".

GnoVM implemented the conversion for the 64-bit-capable kinds (int, int64, uint, uint64) as string(rune(tv.GetInt64())) etc. The rune(...) wrapper truncates to int32 before Go's own range check runs, so an out-of-range value can alias onto a valid code point instead of yielding �:

input	Go	gno (master)
string(uint64(0x10001F600))	�	😀
string(int(-4294967231))	�	A
string(uint64(0x100000000))	�	"\x00"

The fix checks that the value fits in an int32 first and returns utf8.RuneError otherwise; once a value fits in int32, Go's native string(rune) already maps every invalid case (negatives, surrogate halves, > 0x10FFFF) to �. Both the runtime and constant-evaluation paths flow through the same ConvertTo, so one fix covers both (including untyped rune constants like 'A' + 0x100000000).

The remaining integer kinds are intentionally unchanged — int8/int16/uint8/uint16 cannot exceed the int32 range, Int32 converts without narrowing, and uint32 is provably equivalent (values > MaxInt32 reinterpret as negative runes → �; values in (0x10FFFF, MaxInt32] are out of range → �). Details in the included ADR.

Verification

Found via differential testing against the Go toolchain. Verified byte-for-byte against go run across all integer kinds, named types, and untyped constants at every range boundary (0xD7FF/0xD800/0xDFFF/0xE000, 0x10FFFF/0x110000, MaxInt32±1, MinInt32, ±2^32±k, MinInt64/MaxInt64/MaxUint64), in multiple syntactic positions (var init, const decl, concatenation, map keys, switch, struct literals). The new filetest fails on master and passes with the fix; the full TestFiles suite passes.

Note

This PR is AI-assisted (found and developed with Claude); it includes an ADR per AGENTS.md (gnovm/adr/prxxxx_string_conversion_unicode_range.md — to be renamed with the PR number).

Related spec-compliance fixes from the same differential-testing effort: #5784, #5785.

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[Gno2D2]

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[davd-gzl]

Looks good. Verified on b228597: output matches the Go toolchain byte-for-byte across the full boundary table, and reverting the fix brings back the old truncation aliasing (string(uint64(0x10001F600)) gives 😀 instead of �).

Full review: https://github.com/samouraiworld/gno-agent-workspace/blob/main/reviews/pr/5xxx/5807-string-conversion-unicode-range/1-b22859722/review_claude-opus-4-8_davd-gzl.md

(AI Agent)

Related: #5749