Fix flattened child unknown fields

[AllanChain]

Previously, when we have a flattened child class with deny_unknown_fields=True, we will have a problem deserializing.

import serde

@serde.serde(deny_unknown_fields=True)
class A:
    a: int

@serde.serde
class B:
    b: int
    a: A = serde.field(flatten=True, default_factory=A)

serde.from_dict(B, {"a": 1, "b": 2})

Actual behavior:

serde.compat.SerdeError: unknown fields: {'b'}, expected one of {'a'}

Expected:

B(b=2, a=A(a=1))

The fix strategy is passing only the relevant flattened input to child deserializers, achieved by knowing the exact keys expected by the flattened child. Special cases are flattened dict fields. If the parent has flattened dict fields whereas the flattened child does not, we will keep the unknown fields only visible by the parent. Otherwise, we will pass those to the child.

---

The changes are done by a coding agent, and I have reviewed the code by trying to understand the changes line by line. The changes are reasonable and readable.

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.94%. Comparing base (f09d503) to head (fb6def1).
⚠️ Report is 72 commits behind head on main.

Files with missing lines	Patch %	Lines
serde/de.py	93.93%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #752      +/-   ##
==========================================
+ Coverage   90.83%   90.94%   +0.10%     
==========================================
  Files          13       13              
  Lines        2040     2340     +300     
  Branches      370      457      +87     
==========================================
+ Hits         1853     2128     +275     
- Misses        124      140      +16     
- Partials       63       72       +9     

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[yukinarit]

LGTM, thanks!