fix: map missing_required_field to MissingRequiredFieldsError

[mayankbohradev]

Summary

• Register the documented API error name missing_required_field (singular) so it raises MissingRequiredFieldsError.
• Keep missing_required_fields (plural) as a backward-compatible alias.
• Add unit tests for both names.

Why

Resend error docs define missing_required_field (singular). The Python SDK ERRORS map only had missing_required_fields (plural), so a real 422 with name: "missing_required_field" fell through to generic ResendError. Callers catching MissingRequiredFieldsError never matched.

Validation (before / after)

Reproducible with the public mapper (no network):

from resend.exceptions import (
    MissingRequiredFieldsError,
    ResendError,
    raise_for_code_and_type,
)

# Before this change:
#   missing_required_field  -> ResendError
#   missing_required_fields -> MissingRequiredFieldsError
#
# After this change:
#   both names -> MissingRequiredFieldsError

for name in ("missing_required_field", "missing_required_fields"):
    try:
        raise_for_code_and_type(422, name, "Missing `to` field")
    except MissingRequiredFieldsError:
        print(name, "-> MissingRequiredFieldsError")
    except ResendError as e:
        print(name, "->", type(e).__name__)

Expected after this PR:

missing_required_field -> MissingRequiredFieldsError
missing_required_fields -> MissingRequiredFieldsError

Same path as Request.perform / AsyncRequest.perform, which pass data["name"] into raise_for_code_and_type.

Testing

• pytest tests/exceptions_test.py (16 passed)
• mypy resend/exceptions.py tests/exceptions_test.py
• flake8 --max-line-length=130 ./resend/exceptions.py

---

Summary by cubic

Correctly raises MissingRequiredFieldsError for 422 responses with the documented missing_required_field name, so callers catching this exception now match real API errors.

• Bug Fixes
  • Map missing_required_field in the 422 ERRORS table; keep missing_required_fields as a legacy alias.
  • Add unit tests for both names and update the raise_for_code_and_type docstring.

^{Written for commit e3a1e13. Summary will update on new commits.}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.96%. Comparing base (7f1acd0) to head (e3a1e13).
⚠️ Report is 188 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #237       +/-   ##
===========================================
+ Coverage   82.66%   95.96%   +13.29%     
===========================================
  Files           4       61       +57     
  Lines          75     3219     +3144     
===========================================
+ Hits           62     3089     +3027     
- Misses         13      130      +117     

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[felipefreitag]

Thank you @mayankbohradev !