fix(render): encode non-BMP characters as UTF-16 surrogate pairs in AsciiJSON

[amitmishra11]

Bug

AsciiJSON silently corrupts any Unicode code point above U+FFFF (emoji, CJK Extension B, etc.). The escaping loop uses fmt.Appendf(buf, "\u%04x", r), but %04x is a minimum width, not a fixed width. A non-BMP rune produces a 5-digit escape such as ὠ0 instead of the required 4-digit form. JSON parsers read exactly 4 hex digits for \u, so the 5th digit becomes literal text and the decoded string is silently wrong.

Example: rendering the string consisting of U+1F600 (grinning face emoji):

Before fix: AsciiJSON output is {"msg":"ὠ0"}, json.Unmarshal gives "xE0" (wrong)
After fix: AsciiJSON output is {"msg":"😀"}, json.Unmarshal gives the original emoji (correct)

Reported in #4688.

Root cause

Per RFC 8259 section 7, code points outside the BMP must be encoded as a UTF-16 surrogate pair (\uHHHH\uLLLL). The existing code uses a single \uXXXX token regardless of the code point size.

Fix

• Import unicode/utf16.
• In the AsciiJSON.Render loop, add a branch for r > 0xFFFF that calls utf16.EncodeRune and emits two four-digit \uXXXX escapes.
• BMP characters (U+0080-U+FFFF) continue to use a single \uXXXX escape as before.

Test

Added TestRenderAsciiJSONNonBMP in render/render_test.go. It checks several emoji and a CJK Extension B character for:

1. Pure-ASCII output (every byte < 128).
2. Correct json.Unmarshal round-trip (decoded string equals the original input).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (3dc1cd6) to head (ed773a4).
⚠️ Report is 289 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4728      +/-   ##
==========================================
- Coverage   99.21%   98.33%   -0.89%     
==========================================
  Files          42       48       +6     
  Lines        3182     3176       -6     
==========================================
- Hits         3157     3123      -34     
- Misses         17       43      +26     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.31% <100.00%> (?)
-tags go_json	98.25% <100.00%> (?)
-tags nomsgpack	98.30% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.33% <100.00%> (?)
go-1.26	98.33% <100.00%> (?)
macos-latest	98.33% <100.00%> (-0.89%)	⬇️
ubuntu-latest	98.33% <100.00%> (-0.89%)	⬇️

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