[FLINK-38345][flink-connector-base] Unit test to replicate null previousEnumerator for HybridSource on checkpoint recovery

[mattcuento]

What is the purpose of the change

*Adding a unit test to replicate a failure to recover enumerator state in the HybridSourceSplitEnumerator after checkpoint recovery where the underlying source depends on the SourchSwitchContext to be constructed.

This branch does NOT include a fix and is intended for sharing reproductino of this issue.*

Brief change log

(for example:)

• *Replicated failure to recover enumerator state after source switch in the HybridSourceSplitEnumerator

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change added tests and can be verified as follows:

(example:)

• Added unit test in HybridSourceSplitEnumeratorTest class to reproduce failure to create a source that depends on the previousEnumerator via the SourceSwitchContext when recovering from a checkpoint.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 2d67764 Azure: FAILURE

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