core/filtermaps: use buffered channel to avoid blocking

[jsvisa]

found in this job https://github.com/ethereum/go-ethereum/actions/runs/18737728369/job/53448010941?pr=33002

panic: test timed out after 45m0s
	running tests:
		TestIndexerRandomRange (45m0s)

goroutine 23383 [running]:
testing.(*M).startAlarm.func1()
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:2484 +0x394
created by time.goFunc
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/time/sleep.go:215 +0x2d

goroutine 1 [chan receive, 44 minutes]:
testing.(*T).Run(0xc0001ee380, {0x8cab83?, 0xc0001cdb30?}, 0x9bcb30)
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:1859 +0x431
testing.runTests.func1(0xc0001ee380)
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:2279 +0x37
testing.tRunner(0xc0001ee380, 0xc0001cdc70)
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:1792 +0xf4
testing.runTests(0xc0000112c0, {0xded260, 0x8, 0x8}, {0xe34a00?, 0x7?, 0xe324e0?})
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:2277 +0x4b4
testing.(*M).Run(0xc00027c780)
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:2142 +0x64a
main.main()
	_testmain.go:59 +0x9b

goroutine 7 [runnable]:
github.com/ethereum/go-ethereum/core/filtermaps.(*FilterMaps).WaitIdle(0xc003214a08)
	/opt/actions-runner/_work/go-ethereum/go-ethereum/core/filtermaps/indexer.go:147 +0x85
github.com/ethereum/go-ethereum/core/filtermaps.TestIndexerRandomRange(0xc0001ee540)
	/opt/actions-runner/_work/go-ethereum/go-ethereum/core/filtermaps/indexer_test.go:119 +0x425
testing.tRunner(0xc0001ee540, 0x9bcb30)
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:1792 +0xf4
created by testing.(*T).Run in goroutine 1
	/opt/actions-runner/_work/_tool/go/1.24.9/x64/src/testing/testing.go:1851 +0x413

goroutine 23404 [chan send]:
github.com/ethereum/go-ethereum/core/filtermaps.(*FilterMaps).processSingleEvent(0xc003214a08, 0x1?)
	/opt/actions-runner/_work/go-ethereum/go-ethereum/core/filtermaps/indexer.go:206 +0x3db
github.com/ethereum/go-ethereum/core/filtermaps.(*FilterMaps).indexerLoop(0xc003214a08)
	/opt/actions-runner/_work/go-ethereum/go-ethereum/core/filtermaps/indexer.go:51 +0xee
created by github.com/ethereum/go-ethereum/core/filtermaps.(*FilterMaps).Start in goroutine 7
	/opt/actions-runner/_work/go-ethereum/go-ethereum/core/filtermaps/filtermaps.go:292 +0x185
FAIL	github.com/ethereum/go-ethereum/core/filtermaps	2700.047s

There's a race where processSingleEvent would send the idle status, while the caller hadn’t yet reached the receive, deadlocking both goroutines.

So let's make the per-call channel buffered, then the indexer enqueue the ack even if the WaitIdle goroutine hasn’t reached the receive yet.