test: add round-trip tests for collections and structs

[leftygbalogh]

Summary

Adds explicit table-driven round-trip tests for collection types and structs, covering gaps in the current upstream test suite.

Files added

** ests/test_collections.rs** (18 tests)

• Vec round-trips for i32, f64, String, bool
• Empty Vec round-trip to empty Vec (not None)
• Tuple characterisation: (A, B) Python representation observed and asserted
• HashMap<String, V> content equality (no order assertion)
• BTreeMap<String, V> ordering and content equality

** ests/test_structs.rs** (12 tests)

• Basic struct round-trip
• #[serde(rename)] — renamed key present in Python dict, original absent
• #[serde(rename_all = "camelCase")] — all keys renamed
• Unknown fields default behaviour — silently ignored, no panic
• #[serde(deny_unknown_fields)] — returns Err
• Option None and Some variants both round-trip correctly

Test approach

Plain #[test] functions using Python::attach. No new dev-dependencies. All tests pass with cargo test, cargo test --features pyo3/abi3-py37, and cargo test --features arbitrary_precision.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.49%. Comparing base (0085a18) to head (4b76820).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   84.48%   85.49%   +1.01%     
==========================================
  Files           3        3              
  Lines        1186     1186              
  Branches     1186     1186              
==========================================
+ Hits         1002     1014      +12     
+ Misses        118      106      -12     
  Partials       66       66              

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[davidhewitt]

Thanks