[FLINK-36236] Fix overriding the connection provider in JdbcSourceBuilder

[rajucomp]

FLINK-36236: Fix Overriding the Connection Provider in JdbcSourceBuilder

Summary

This PR fixes an issue where a custom JdbcConnectionProvider passed via JdbcSourceBuilder#setConnectionProvider was unintentionally overwritten during the build() process. This prevented users from injecting their own connection management logic (e.g., for testing or advanced connection pooling scenarios).

Changes

• JdbcSourceBuilder

  • Modified build() method to only set a default SimpleJdbcConnectionProvider if no custom provider has been set.

• JdbcSource

  • Exposed a getConnectionProvider() method with @VisibleForTesting to enable access from test code.

• Test Coverage

  • Added a unit test testSetConnectionProvider() to:
    • Verify that null values are rejected.
    • Assert that the JdbcSource retains and uses the explicitly provided JdbcConnectionProvider.

Motivation

This change ensures the builder pattern behaves predictably: user-provided values should not be silently overridden. This is especially important when dependency injection or custom behavior is expected during source initialization.

[boring-cyborg]

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[rajucomp]

@eskabetxe Can I get a review please ? Thanks.

[eskabetxe]

LGTM.

My only concern is the change in visibility of some methods in a @PublicEvolving class. I understand they're annotated with @VisibleForTesting, but we should be cautious with such changes.

@1996fanrui can you run CI/CD please

[rajucomp]

@@eskabetxe the checks have passed. Could you merge the PR ? I don't think I have the permission to do so. Thanks!

[ferenc-csaky]

LGTM! Based on the definition (pasted below) @PublicEvolving I see no harm in restricting the visibility of those getters.

Classes and methods with this annotation are intended for public use and have stable behavior. However, their interfaces and signatures are not considered to be stable and might be changed across versions. 

@rajucomp this supersedes #150, right?

[ferenc-csaky]

Anyways, will merge this by the end of today if no objections until then, this is a fairly simple change and it is out there for quite a while already.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!