test: add discard output unit coverage

[github-actions]

Summary

This PR adds focused unit tests for libbeat/outputs/discard to cover behavior that was previously untested:

• Publish ACK behavior and observer accounting.
• invalid queue configuration handling in makeDiscard.
• bulk_max_size override behavior in makeDiscard.

User scenarios

• When a user sets output.discard to benchmark or debug pipeline behavior, every emitted event must still be ACKed by the output path so the pipeline can progress correctly.
• When a user configures output.discard.queue with an invalid queue type, startup should fail with a clear error instead of silently accepting invalid configuration.
• When a user enables output.discard, the output must disable batching (bulk_max_size = -1) so publisher behavior is consistent with this sink.

Why these tests matter

• They catch regressions where discard output stops ACKing batches or misreports metrics.
• They guard config validation/error propagation during output initialization.
• They protect a key runtime contract (bulk_max_size=-1) used by pipeline behavior.

Tests run

• go test ./libbeat/outputs/discard ✅
• go test -count=5 ./libbeat/outputs/discard ✅ (stability/repeatability check)

Removed or merged tests

• None.

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What is this? | From workflow: Test Improver

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