[SharovBot] fix: track MergeLoop goroutine in bgComponentsEg to prevent data race on shutdown

[erigon-copilot]

[SharovBot]

Problem

A data race was detected in TestImportClosesChaindataOnInitError (race-tests CI job, 2026-07-03):

WARNING: DATA RACE
Write at 0x00c0653acb90 by goroutine 321:
  github.com/erigontech/erigon/db/state.(*Aggregator).Close()
      db/state/aggregator.go:630

Previous read at 0x00c0653acb90 by goroutine 373:
  github.com/erigontech/erigon/db/state.(*Aggregator).MergeLoop()
      db/state/aggregator.go:1228
  github.com/erigontech/erigon/node/eth.New.func16()
      node/eth/backend.go:1133

Context

PR #22203 (merged 2026-07-04) addressed this race by replacing sync.WaitGroup with a closingWaitGroup latch in Aggregator, making MergeLoop's TryAdd() properly ordered against Close()'s BeginClose()+Wait().

This PR

This PR provides an additional, complementary fix: track the MergeLoop goroutine in bgComponentsEg so Stop() → bgComponentsEg.Wait() explicitly waits for the MergeLoop goroutine to exit before chainDB.Close() is called.

Without this, bgComponentsEg.Wait() in Stop() returns without waiting for the MergeLoop goroutine (since it was launched as a bare go func()), meaning the goroutine could theoretically still be running when chainDB.Close() begins. The closingWaitGroup handles the WaitGroup reuse race, but this PR makes the shutdown ordering explicit and unambiguous.

Changes:

• Moves the MergeLoop goroutine from a bare go func() to backend.bgComponentsEg.Go()
• Fixes the typo: "snapashot" → "snapshot" in the error message
• Filters context cancellation errors (expected on shutdown) from logger.Error

Testing

• go test -race -count=10 ./cmd/utils/app/ -run TestImportClosesChaindataOnInitError — all 10 runs PASS, no data race
• go build ./... — succeeds
• No test files modified

Fixes CI: https://github.com/erigontech/erigon/actions/runs/28674562917/job/85045122311