Fix enum expr for enum selector

[goccy]

Fix code generation issue when using grpc.federation.enum.select in enum {} or map { enum {} } feature.

grpc.federation.enum.select determines the enum type at runtime. The evaluation of the enum type occurs when using the def.enum.by feature or performing field binding. In all other cases, it is treated as an EnumSelector type without further processing.

[github-actions]

Code Metrics Report

	main (a603e99)	#261 (cf48ed6)	+/-
Coverage	66.1%	66.2%	+0.2%
Code to Test Ratio	1:0.4	1:0.4	-0.0
Test Execution Time	8m4s	8m17s	+13s

Details

  |                     | main (a603e99) | #261 (cf48ed6) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          66.1% |          66.2% | +0.2% |
  |   Files             |             77 |             77 |     0 |
  |   Lines             |          13129 |          13160 |   +31 |
+ |   Covered           |           8673 |           8715 |   +42 |
- | Code to Test Ratio  |          1:0.4 |          1:0.4 |  -0.0 |
  |   Code              |          40671 |          40739 |   +68 |
+ |   Test              |          15724 |          15732 |    +8 |
- | Test Execution Time |           8m4s |          8m17s |  +13s |

Code coverage of files in pull request scope (73.8% → 74.0%)

Files	Coverage	+/-
generator/code_generator.go	77.9%	-0.4%
grpc/federation/cel.go	0.0%	0.0%
resolver/message.go	76.7%	+0.6%
resolver/resolver.go	84.0%	+0.5%
resolver/type_conversion.go	82.4%	-1.8%
resolver/types.go	87.5%	+0.8%

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Reported by octocov