Serialize a dataclass schema to a field list

[farmio]

Breaking change

None. A dataclass schema raised before, so nothing that worked changes. Nested
mappings still raise.

Proposed change

to_field_list unwrapped a Schema to its node. For a dataclass schema that node
is the validate-then-construct pair, not the mapping, so it raised while the
equivalent TypedDictSchema serialized fine.

constructed_mapping() in dataclass_schema.py returns the mapping a
constructing schema validates, and to_field_list uses it before serializing.
Putting the check there keeps the shape of the pair private to the module that
builds it, rather than teaching the codec how a dataclass schema is assembled.

Nested mappings are left alone: voluptuous-serialize raises for those too, and a
test now pins both sides of that.

One open question for you. I added constructed_mapping as a module-level
function because create_dataclass_schema returns a plain Schema, so a method
or property on DataclassSchema would not cover the functional builder. If you
would rather not grow the module surface, the alternative is a shape check inside
the codec, which I liked less.

fixes #313

Type of change

• Bugfix (non-breaking change that fixes an issue)

Additional information

• This PR fixes or closes issue: fixes to_field_list cannot serialize a dataclass schema #313

Checklist

• I fully understand the code in this pull request and can explain every line, including any AI-assisted changes.
• The change is covered by tests, and uv run --no-sync just test passes locally.
• uv run --no-sync just lint passes.
• uv run --no-sync just typecheck passes.
• No commented-out or dead code is left in the pull request.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 23 untouched benchmarks

---

_{Comparing farmio:fix/serialize-nested-schema-nodes (756f732) with main (afa4b7a)}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (afa4b7a) to head (756f732).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files           45        45           
  Lines         6590      6596    +6     
  Branches      1220      1221    +1     
=========================================
+ Hits          6590      6596    +6     

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