Dictionary entries lose their keys and collapse into one after reload when the key enum uses a 64-bit backing type

Steps to reproduce:

1. Open the attached “IN-150204” project
2. Wait for the project to finish compiling
3. Go to Tools > Reproduce 64-bit Enum Dictionary Bug
4. Open dict-roundtrip-result.txt in the project root
5. Observe the result

Actual result: The 64-bit enum dictionary changes from count=2 {Alpha=10, Huge=20} to count=1 {0=10} after reload. The serialized YAML contains the values but omits their keys
Expected result: The 64-bit enum dictionary is serialized correctly, or Unity rejects the unsupported field before saving without losing entries

Reproducible with: 6000.6.0b6, 6000.6.2f1, 6000.7.0b1, 7000.0.0a1
Could not test with: 6000.0.0f1, 6000.0.84f1, 6000.3.24f1 (Native Dictionary serialization is unavailable)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Note: Dictionaries using 32-bit or smaller enum keys and string keys serialize correctly. Some versions log a duplicate-key warning, but the entries are still lost