test: add error-path and asymmetry characterisation tests

[leftygbalogh]

Summary

Adds characterisation and error-path tests covering type-mismatch cases and asymmetric round-trip behaviours.

Files added

tests/test_error_paths.rs (8 tests)

• HashMap<i64, V> serialized by pythonize -> Ok (unexpected: DictKeyNotString lives in the deserializer only; see note below)
• HashMap<bool, V> -> same root cause
• Python str -> depythonize::<i64> -> Err
• Python bytes -> depythonize::<String> -- characterised
• Python None / list -> depythonize::<i64> -> Err

tests/test_asymmetry.rs (16 tests)

• BTreeMap content equality after round-trip
• f32 precision: MIN_POSITIVE, PI, subnormal -- all survive; // BASELINE: comments note the f64 widening
• char length guard: "ab" and "hello" -> Err (correct rejection)
• bool -> i64/i32: Ok(1)/Ok(0) -- Python bool subclasses int
• i64(300) -> i8/u8 overflow -> Err (no silent truncation)
• f64::NAN -> Python -> f32: Ok(NaN) -- NaN survives narrowing
• Unit () round-trip: Ok

Notable findings

Non-string map keys serialize without error (HashMap<i64, V>, HashMap<bool, V>): pythonize produces a Python dict with non-string keys -- the DictKeyNotString check exists only in the deserializer. The round-trip breaks silently on the deserialization side. Tests assert the observed Ok and are flagged // BASELINE: UNEXPECTED.

Test approach

Plain #[test] functions using Python::attach. No new dev-dependencies. Characterisation tests use // BASELINE: comments; surprising cases are additionally flagged // BASELINE: UNEXPECTED.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.49%. Comparing base (0085a18) to head (e3830df).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   84.48%   85.49%   +1.01%     
==========================================
  Files           3        3              
  Lines        1186     1186              
  Branches     1186     1186              
==========================================
+ Hits         1002     1014      +12     
+ Misses        118      107      -11     
+ Partials       66       65       -1     

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[davidhewitt]

What are these AC-1, AC-2 notes all over the files?