feat(protoenc): decode UDT type anchors

[benbellick]

UDT serialization now assigns anchors from semantic extension type IDs, but the decode path also needs registry context to recover those IDs from protobuf anchors.

This adds the registry-aware type decode path and uses it from expression and literal decoding paths that already have an extension registry available.

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Note: This PR was developed with AI assistance. All changes have been reviewed, and I take full responsibility for this contribution.

[codecov]

Codecov Report

❌ Patch coverage is 35.52632% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.08%. Comparing base (04903bb) to head (745b829).

Files with missing lines	Patch %	Lines
expr/literals.go	21.42%	31 Missing and 2 partials ⚠️
expr/expression.go	55.17%	7 Missing and 6 partials ⚠️
expr/functions.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           refactor/preserve-testcase-udt-names     #263      +/-   ##
========================================================================
- Coverage                                 70.30%   70.08%   -0.22%     
========================================================================
  Files                                        48       48              
  Lines                                     11044    11099      +55     
========================================================================
+ Hits                                       7764     7779      +15     
- Misses                                     2910     2941      +31     
- Partials                                    370      379       +9     

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