fix(serde): StreamDeserializer ends without an error after the last value

[yuxi-liu-wired]

fix: StreamDeserializer ends without an error after the last value

into_stream() yielded an extra Err(EOF while parsing) once the input
was exhausted, so every well-formed stream ended in an error and a loop
like for v in stream { v? } always failed:

Deserializer::from_str("1 2").into_stream::<Value>()
    Ok(1), Ok(2), Err(EOF while parsing at line 1 column 3)
Deserializer::from_str("").into_stream::<Value>()
    Err(EOF while parsing at line 1 column 1)

serde_json's StreamDeserializer returns None there, which issue #202
asks sonic-rs to align with. next() now checks for remaining non-space
input first; a value that is cut short or garbage after a value is
still an error and still ends the stream.

(serde_json additionally requires whitespace between two values that do
not delimit themselves, e.g. 1true; sonic-rs splits them. Not changed
here.)

Test: test_stream_deserializer_ends_without_error (fails before this
change).

Testing

Each commit adds a regression test that fails before the change. The full test suite passes and
cargo fmt --check is clean on the changed files.

Found with a differential fuzzer comparing serde_json, simd-json, sonic-rs and jiter.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.36%. Comparing base (9de5a49) to head (3787849).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
+ Coverage   73.08%   73.36%   +0.28%     
==========================================
  Files          42       42              
  Lines       10007    10030      +23     
==========================================
+ Hits         7314     7359      +45     
+ Misses       2693     2671      -22     

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