MINOR: Use TransactionOperation enum instead of String for pending state checks

[chickenchickenlove]

PR Description

this PR refactors TransactionManager#throwIfPendingState to use a typed TransactionOperation enum instead of a raw String.

###Background / Motivation
The previous String-based API relies on manually passing operation names, which can make it easier for naming to drift or become inconsistent over time. Using an enum makes the intent more explicit and keeps the operation naming centralized.

What’s changed

• Added a TransactionOperation enum with a displayName (and toString() override) to preserve readable exception messages.
• Updated relevant call sites (beginTransaction, prepareTransaction, sendOffsetsToTransaction, maybeAddPartition) to pass the corresponding enum value.
• Updated throwIfPendingState to accept TransactionOperation instead of String.

Behavior / Compatibility

• No behavioral change is intended. The exception message remains human-readable and consistent via TransactionOperation#toString().

Testing

No additional tests were added since this is a small refactor; existing tests should continue to cover the pending-transition behavior.

[TaiJuWu]

LGTM.

[chickenchickenlove]

Hi, @chia7712 !
Sorry to bother you and thanks for your approving CI 🙇‍♂️
When you have time, could you take a look?
Thanks always!