test: add core ingest coverage with httptest server

[acoshift]

Summary

Adds dedicated tests for the core ingest feature, exercised against an httptest server so no real Quickwit instance is required. Complements the existing regression tests by covering the happy-path mechanics and operational behaviors of the buffered ingest pipeline.

New file ingest_test.go (black-box quickwit_test package, matching existing style):

Test	What it verifies
TestIngest_DeliversAllItemsInOrder	All records reach the server exactly once, in order, across multiple batches
TestIngest_FlushTriggeredByBatchSize	Reaching the batch size flushes immediately (timer pinned far out)
TestIngest_FlushTriggeredByTimer	A partial batch flushes on the timer (batch size never reached)
TestIngest_FlushRemainingOnClose	Close() flushes whatever is still buffered
TestIngest_RetriesFailedFlushUntilSuccess	A failing flush retries until the server accepts it; no data loss
TestIngest_AppliesAuthHook	The user auth hook is applied to ingest requests
TestIngest_ConcurrentWorkersDeliverEachItemOnce	With multiple workers every record is delivered exactly once (no loss/dup)
TestIngest_DiscardsWhenBufferFull	In discard mode, overflow items are dropped via OnDiscard instead of blocking
TestIngest_413ReducesBatchSizeAndChunks	A 413 shrinks the batch and re-sends in smaller chunks, still in order

Test plan

• go vet ./...
• go test -race -count=5 ./... — passes (timing-sensitive tests stable across runs)

🤖 Generated with Claude Code