fix: decode nested msgspec rename fields

[cofin]

Closes #679

Summary

• replace root-level msgspec rename inference with cached, type-aware input normalization based on exact Struct field encode names
• normalize nested Structs through declared optional, annotated, collection, tuple, and mapping types without rewriting arbitrary mapping keys
• preserve encoded aliases, prefer encoded keys when both aliases are present, and leave unknown fields and ambiguous unions to msgspec validation
• add regression coverage for single records, batches, mixed rename conventions, callable and explicit aliases, nested optionals and collections, duplicate aliases, arbitrary mappings, and strict unknown-field handling

Root cause

The conversion path inferred one rename convention from the outer Struct and recursively applied it to every nested dictionary. Single-word outer fields did not reveal the configured convention, while mixed nested conventions and arbitrary dictionary payloads could not be represented safely by one global transform.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.73239% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.38%. Comparing base (b6b946a) to head (376a2f1).

Files with missing lines	Patch %	Lines
sqlspec/utils/schema.py	88.73%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #680      +/-   ##
==========================================
+ Coverage   77.32%   77.38%   +0.05%     
==========================================
  Files         476      476              
  Lines       68107    68163      +56     
  Branches     9363     9376      +13     
==========================================
+ Hits        52665    52746      +81     
- Misses      12033    12035       +2     
+ Partials     3409     3382      -27     

Flag	Coverage Δ
py3.10	75.66% <88.73%> (+0.01%)	⬆️
py3.12	75.67% <88.73%> (+0.01%)	⬆️
py3.13	75.67% <88.73%> (+0.01%)	⬆️
py3.14	76.55% <88.73%> (+0.01%)	⬆️
unit	64.92% <88.73%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sqlspec/utils/schema.py	91.53% <88.73%> (+7.56%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.