[Go] Serialize readonly fields

[gcatanese]

PR #14335 has mistakenly removed the readOnly fields during the JSON serialization, resulting in models with missing fields.

This PR reverts those changes.

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[gcatanese]

@wing328 Please see this PR that corrects the bug introduced by #14335

[wing328]

thanks for the fix. can you please add a test in samples/openapi3/client/petstore/go/model_test.go to cover this issue moving forward?

[gcatanese]

Sure. I have never noticed that before. When do those tests run?

[gcatanese]

New test added in model_test.go

[wing328]

thanks for adding the test but CI tests failed:

=== RUN   TestReadOnlyFirst
    model_test.go:52: 
        	Error Trace:	/home/circleci/OpenAPITools/openapi-generator/samples/openapi3/client/petstore/go/model_test.go:52
        	Error:      	Not equal: 
        	            	expected: *testing.T(&testing.T{common:testing.common{mu:sync.RWMutex{w:sync.Mutex{state:0, sema:0x0}, writerSem:0x0, readerSem:0x0, readerCount:0, readerWait:0}, output:[]uint8(nil), w:testing.indenter{c:(*testing.common)(0xc0001b8820)}, ran:false, failed:false, skipped:false, done:false, helperPCs:map[uintptr]struct {}{0x6d89f3:struct {}{}, 0x6d9cd4:struct {}{}}, helperNames:map[string]struct {}(nil), cleanups:[]func()(nil), cleanupName:"", cleanupPc:[]uintptr(nil), finished:false, inFuzzFn:false, chatty:(*testing.chattyPrinter)(0xc00007b710), bench:false, hasSub:0, raceErrors:0, runner:"testing.tRunner", parent:(*testing.common)(0xc0001111e0), level:1, creator:[]uintptr{0x4e70ee, 0x4e43a2, 0x4e6f97, 0x4e5a79, 0x6e5045, 0x6e9093, 0x439a52, 0x469101}, name:"TestReadOnlyFirst", start:time.Time{wall:0xc1182cac151656b8, ext:21342773301, loc:(*time.Location)(0x9df6c0)}, duration:0, barrier:(chan bool)(0xc0000c69c0), signal:(chan bool)(0xc0000d6690), sub:[]*testing.T(nil), tempDirMu:sync.Mutex{state:0, sema:0x0}, tempDir:"", tempDirErr:error(nil), tempDirSeq:0}, isParallel:false, isEnvSet:false, context:(*testing.testContext)(0xc0000785a0)})
        	            	actual  : string("{\"Bar\":\"Bar value\",\"Baz\":\"Baz value\"}")
        	Test:       	TestReadOnlyFirst
        	Messages:   	{"bar":"Bar value","baz":"Baz value"}
--- FAIL: TestReadOnlyFirst (0.00s)
=== RUN   TestNullableMarshalling

can you please take a look when you've time?

https://app.circleci.com/pipelines/github/OpenAPITools/openapi-generator/24086/workflows/622226ea-baf9-4c39-8561-fa768ab20b1f/jobs/70266

[gcatanese]

@wing328 I have fixed the test, thank you

[wing328]

cc @antihax (2017/11) @grokify (2018/07) @kemokemo (2018/09) @jirikuncar (2021/01) @ph4r5h4d (2021/04) @lwj5 (2023/04)