feat(python): Add enumUnknownDefaultCase support for backward compatibility

[ranjanp75]

This PR adds support for the enumUnknownDefaultCase option to the Python generator, enabling graceful handling of unknown enum values. This feature improves backward compatibility when servers introduce new enum values that older clients don't recognize.

Fixes #20012

Usage:
--additional-properties enumUnknownDefaultCase=true

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• Read the contribution guidelines.
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  ./mvnw clean package || exit
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  (For Windows users, please run the script in WSL)
  Commit all changed files.
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@wing328 @spacether @cbornet @tomplus @krjakbrjak @fa0311 @multani

[ranjanp75]

@wing328 @spacether @cbornet @tomplus @krjakbrjak @fa0311 @multani Please review. Thanks

[wing328]

please review the build failure when you've time:

Error:  org.openapitools.codegen.python.PythonClientCodegenTest.testEnumModelWithUnknownDefaultCase -- Time elapsed: 0.004 s <<< ERROR!
java.lang.NullPointerException
	at org.openapitools.codegen.utils.ModelUtils.getOpenApiVersion(ModelUtils.java:2034)
	at org.openapitools.codegen.TestUtils.parseSpec(TestUtils.java:66)
	at org.openapitools.codegen.TestUtils.parseFlattenSpec(TestUtils.java:45)
	at org.openapitools.codegen.python.PythonClientCodegenTest.testEnumModelWithUnknownDefaultCase(PythonClientCodegenTest.java:710)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.testng.internal.invokers.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:141)
	at org.testng.internal.invokers.TestInvoker.invokeMethod(TestInvoker.java:686)
	at org.testng.internal.invokers.TestInvoker.invokeTestMethod(TestInvoker.java:230)
	at org.testng.internal.invokers.MethodRunner.runInSequence(MethodRunner.java:63)
	at org.testng.internal.invokers.TestInvoker$MethodInvocationAgent.invoke(TestInvoker.java:992)
	at org.testng.internal.invokers.TestInvoker.invokeTestMethods(TestInvoker.java:203)
	at org.testng.internal.invokers.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:154)
	at org.testng.internal.invokers.TestMethodWorker.run(TestMethodWorker.java:134)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
	at org.testng.TestRunner.privateRun(TestRunner.java:739)
	at org.testng.TestRunner.run(TestRunner.java:614)
	at org.testng.SuiteRunner.runTest(SuiteRunner.java:421)
	at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:413)
	at org.testng.SuiteRunner.privateRun(SuiteRunner.java:373)
	at org.testng.SuiteRunner.run(SuiteRunner.java:312)
	at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
	at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:95)
	at org.testng.TestNG.runSuitesSequentially(TestNG.java:1274)
	at org.testng.TestNG.runSuitesLocally(TestNG.java:1208)
	at org.testng.TestNG.runSuites(TestNG.java:1112)
	at org.testng.TestNG.run(TestNG.java:1079)

Error:  Errors: 
Error:  org.openapitools.codegen.python.PythonClientCodegenTest.testEnumGenerationWithUnknownDefaultCase
Error:    Run 1: PythonClientCodegenTest.testEnumGenerationWithUnknownDefaultCase:771 » NullPointer
Error:    Run 2: PythonClientCodegenTest.testEnumGenerationWithUnknownDefaultCase:771 » NullPointer
Error:    Run 3: PythonClientCodegenTest.testEnumGenerationWithUnknownDefaultCase:771 » NullPointer
Error:  org.openapitools.codegen.python.PythonClientCodegenTest.testEnumGenerationWithoutUnknownDefaultCase
Error:    Run 1: PythonClientCodegenTest.testEnumGenerationWithoutUnknownDefaultCase:798 » NullPointer
Error:    Run 2: PythonClientCodegenTest.testEnumGenerationWithoutUnknownDefaultCase:798 » NullPointer
Error:    Run 3: PythonClientCodegenTest.testEnumGenerationWithoutUnknownDefaultCase:798 » NullPointer
Error:  org.openapitools.codegen.python.PythonClientCodegenTest.testEnumModelWithUnknownDefaultCase

[ranjanp75]

@wing328 I have fixed the tests, Please re-trigger the builds. Thanks

[wing328]

i enabled the option in one of the samples but the test failed:

#22048

can you please review?