Fix a bug that maxConnections is Short in config but read back as Integer

[olalamichelle]

Also added Unit Tests.

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[bvolpato]

@Abacn Can you please take a look? You have more context here.

I can see that maxConnections is FieldType.INT16, but in the JdbcIO it's an Integer. Maybe we should make it INT32 instead?

[bvolpato]

By the way, thanks for contributing @olalamichelle! I'm pinging Yi as he has a lot of context around this provider

[bvolpato]

There's a check failing for Spotless, please make sure you run ./gradlew spotlessApply to fix formatting

[olalamichelle]

Done.

[Abacn]

Thanks. a previous fix done in #26480 still missing this particular code path (it fixed Python xlang jdbc schemaio but misses java schemaio itself)

[Abacn]

@Abacn Can you please take a look? You have more context here.

I can see that maxConnections is FieldType.INT16, but in the JdbcIO it's an Integer. Maybe we should make it INT32 instead?

Yeah, the bug indicates the usage of INT16 is error prone -- however, change it to INT32 is equivalently error prone either. Many places need to be in sync