fix(serde): correct Value end offset with utf8_lossy and invalid UTF-8

[yuxi-liu-wired]

fix(utf8_lossy): Value end offset when the input has invalid UTF-8

With utf8_lossy, deserializing a Value from input that contains
invalid UTF-8 parses a lossy copy (String::from_utf8_lossy), where
each invalid sequence became the 3-byte U+FFFD. The end offset n
returned for that copy was then used as an offset into the original
input (read.eat(n)), so:

• valid input failed. With the utf8_lossy feature on, plain
  from_slice::<Value>(b"[\"a\xa0b\"]") returned "Invalid UTF-8
  characters" (the documented use of the feature), and so did
  Deserializer::utf8_lossy() + deserialize::<Value>() + end().
• trailing characters were skipped: [\"a\xa0b\"]xx was accepted by
  end(), because the offset overshot the real end.

Deserializing the same input into any other type (String, Vec,
serde_json::Value, ...) was right.

The consumed length in the original is now its length minus what the
parser left unread in the copy (the tail after the value is identical
in both); an overshoot into the padding (unterminated input) is kept,
so it is still EOF. The next-invalid-UTF-8 position is then recomputed
from that point, as the invalid UTF-8 inside the value was replaced.

Test: test_utf8_lossy_value_trailing (fails before this change).
Suite: 249 (default), 246 (utf8_lossy, main 245), 248 (all features,
main 247); Miri clean on the utf8_lossy tests.

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 72.83%. Comparing base (c34a535) to head (5472469).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #253      +/-   ##
==========================================
+ Coverage   72.73%   72.83%   +0.09%     
==========================================
  Files          42       42              
  Lines        9944     9979      +35     
==========================================
+ Hits         7233     7268      +35     
  Misses       2711     2711              

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