[FLINK-30514] Fix HybridSourceReader's method calling order for underlying subsources during source switch

[shanzi]

What is the purpose of the change

This MR is intended to fix the method calling order of the HybridSourceReader described in FLINK-30514. We found it not only affects the File connector but also the Iceberg connector.

In our usage, we'd like to use Iceberg Connector as the first source of the HybridSource, however, we found due to the HybridSource is calling start before addSplits(which will be called during recovery), it breaks IcebergSourceSplitReader's assumption: the recovered splits will be added before start.

In the start method, the IcebergSourceSplitReader with check if there is recovered splits on start, and decide if to request new splits. The wrong order in the HybridSource will lead to a undesired requests of new splits, which breaks watermark, slows down the reading and endangers the stability of a job.

Brief change log

• Fixed the order of method call: calling start() after the recovered splits were added to the reader first.
• Updated unit tests to test the behavior.

Verifying this change

Unit tests was updated to verify the behavior.

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

• The behavior of HybridSource on switching sources should have been fixed to match the correct order.

Documentation

• Does this pull request introduce a new feature? (no)

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[flinkbot]

CI report:

• 2463ee9 Azure: SUCCESS

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[shanzi]

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[shanzi]

cc @tweise @WencongLiu @xccui Can you have a look given you contributed to this file.