Fix processing bug

[eminano]

This PR fixes a bug in the WAL processors where the send/notify channels are closed when the enqueuing fails for any reason, but it should only be closed when the send/notify thread stops (this signals the processor has stopped).

This can cause issues if the error returned by the ProcessWALEvent method is not treated as fatal, and the processor is called after an error, attempting to send a message on a closed channel with a send/notify thread still running.

Preventative nil checks added for the search indexer message.

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/xataio/pgstream/pkg/wal/processor/kafka	71.23% (ø)
github.com/xataio/pgstream/pkg/wal/processor/search	85.83% (ø)
github.com/xataio/pgstream/pkg/wal/processor/webhook/notifier	86.09% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/xataio/pgstream/pkg/wal/processor/kafka/wal_kafka_batch_writer.go	76.47% (ø)	119	91	28
github.com/xataio/pgstream/pkg/wal/processor/search/search_batch_indexer.go	87.40% (ø)	127	111	16
github.com/xataio/pgstream/pkg/wal/processor/search/search_msg_batch.go	88.24% (ø)	17	15	2
github.com/xataio/pgstream/pkg/wal/processor/webhook/notifier/webhook_notifier.go	84.78% (ø)	92	78	14

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.