Fix cast process for cel plugin's list type argument

[goccy]

Fix {"time":"2025-02-20T15:45:07.660496+09:00","level":"ERROR","msg":"internal error: interface conversion: *pluginpb.Example is not ref.Val: missing method ConvertToNative"} error

[github-actions]

Code Metrics Report

	main (158929c)	#274 (67017b2)	+/-
Coverage	65.3%	65.3%	-0.1%
Code to Test Ratio	1:0.3	1:0.3	-0.1
Test Execution Time	7m18s	6m55s	-23s

Details

  |                     | main (158929c) | #274 (67017b2) |  +/-  |
  |---------------------|----------------|----------------|-------|
- | Coverage            |          65.3% |          65.3% | -0.1% |
  |   Files             |             77 |             77 |     0 |
  |   Lines             |          13796 |          13800 |    +4 |
  |   Covered           |           9017 |           9017 |     0 |
- | Code to Test Ratio  |          1:0.3 |          1:0.3 |  -0.1 |
  |   Code              |          42274 |          42279 |    +5 |
  |   Test              |          15831 |          15831 |     0 |
+ | Test Execution Time |          7m18s |          6m55s |  -23s |

Code coverage of files in pull request scope (0.0% → 0.0%)

Files	Coverage	+/-
grpc/federation/cel/plugin.go	0.0%	0.0%

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Reported by octocov