fix: handle unknown topic etc.

[ibukanov]

Refactor processMessagesIntoBatchPipeline to ensure that it closes the message channel to signal that the message processing is ready including the case when the message topic does not match any of configured topics. For that move all processing of the message including the search for topic processors to runMessageProcessor() groutine and use defer to close the channel. That required to change the message channel to be of struct{} type and store the error in the message itself so a simple defer close(msg.done) can be used to cover both normal and error cases.

Provide unit tests for processMessagesIntoBatchPipeline that cover success and error paths.

Bound batchPipeline capacity by the number of CPU cores to avoid running too many CPU-intensive tasks in parallel.

Closes #702 Closes #703 Closes #704

[github-actions[bot]]

[puLL-Merge] - brave-intl/challenge-bypass-server@708

Description

This PR makes several changes to the Kafka package to improve performance, simplify error handling, and make the code more testable.

The main changes are:

• Simplify the Processor function type and remove the ProcessingResult struct
• Add a messageReader interface for testing
• Refactor processMessagesIntoBatchPipeline to limit concurrency based on available CPU cores and simplify message passing
• Add unit tests for processMessagesIntoBatchPipeline
• Refactor error handling in the token issuer and redeem handlers to return errors instead of handling emission directly
• Pass context to Emit and processor functions

Possible Issues

• The changes to limit batchPipeline capacity based on CPU cores assumes the message processors are CPU-bound. If they start doing more I/O, the capacity may need to be adjusted.
• The handlePermanentIssuanceError and handlePermanentRedemptionError functions no longer return after emitting errors. This should be reviewed to ensure it matches the desired semantics for handling permanent errors.

Changes

### Changes kafka/main.go - Simplify `Processor` function signature - Add `messageReader` interface - Refactor `processMessagesIntoBatchPipeline` to limit concurrency and simplify message passing - Update `readAndCommitBatchPipelineResults` to match simplified `MessageContext` - Pass context to `Emit` kafka/main_test.go - Add unit tests for `processMessagesIntoBatchPipeline` kafka/signed_blinded_token_issuer_handler.go - Update handler to match new `Processor` signature - Refactor error handling to return errors - Pass context to error handling function - Return errors from `handlePermanentIssuanceError` kafka/signed_token_redeem_handler.go - Update handler to match new `Processor` signature - Refactor error handling to return errors - Pass context to error handling function - Return errors from `handlePermanentRedemptionError`