json-fuzz: prevent large size inputs

[adisuissa]

Commit Message: json-fuzz: prevent large size inputs
Additional Description:
The original fuzzer test-case created a large input file with ~1MiB of base64 contents that was truncated in the middle and caused the fuzzer to OOM due to a test-function allocation:

#9 0x5ca72f569401 in testing::internal::edit_distance::CreateUnifiedDiff(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>> const&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>> const&, unsigned long) /proc/self/cwd/external/com_google_googletest/googletest/src/gtest.cc:1352:39

This PR limits the input sizes for the json input to 32KiB, as most errors should be detected with this limit.

Risk Level: low - tests only
Testing: N/A
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A
Fixes fuzz issue 421951268.

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🐱

Caused by: #40855 was opened by adisuissa.

see: more, trace.

[botengyao]

Thanks for addressing this!

[botengyao]

lgtm, thanks!