Indexes: Harden the flush-error notification invariant #35731

This is a follow-up to #35714 as it has the two commits of that PR, do not merge or review before that PR

#35714 makes FlushStateToDisk() return state.Error(...) when FlushChainstateBlockFile() fails, relying on the comment // FlushChainstateBlockFile() already emitted the flush-error notification.

That invariant holds today, but it is not guaranteed by construction: it depends on every success = false branch inside FlushBlockFile/FlushUndoFile emitting the notification on its own. A future flush source (a new file type, a new error branch) that sets success = false without notifying would make FlushStateToDisk() return an error without AbortNode().

This PR makes sure that whenever a flush reports failure, the flush-error notification has been emitted:

• test: cover FlushBlockFile flush-error notifications: lower-level BlockManager tests (as suggested in the #35714 review) that inject a failure on the block file and on the undo file separately and assert each failure emits flushError.
• blockstorage: encapsulate flush+notify in FlushFile: a single FlushFile() helper couples the flush and the notification; FlushBlockFile/FlushUndoFile delegate to it. Behaviour is unchanged.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35731.

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Reviews

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35307 (blockstorage: keep snapshot base in normal blockfile range by shuv-amp)
• #35003 (validation: improve block data I/O error handling in P2P paths by furszy)
• #30342 (kernel, logging: Pass Logger instances to kernel objects by ryanofsky)
• #29700 (kernel, refactor: return error status on all fatal errors by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🚧 At least one of the CI tasks failed. _{Task iwyu: https://github.com/bitcoin/bitcoin/actions/runs/29437114552/job/88325958979} _{LLM reason (✨ experimental): CI failed because IWYU reported required header/include fixes (it modified src/node/blockstorage.h and exited non-zero).}

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