change: storage traits return io::Error

[drmingdrmer]

Changelog

change: storage traits return io::Error

Issue #1457: Simplify storage trait error API.

Benefits: Simpler trait implementation - Storage code doesn't need to
know about StorageError<C> or generic type parameters in error handling.

Changes

Storage trait methods now return io::Error instead of StorageError<C>:

• RaftLogStorage: append(), truncate(), purge(), save_vote(), save_committed()
• RaftLogReader: try_get_log_entries(), limited_get_log_entries(), read_vote(), get_key_log_ids()
• RaftStateMachine: applied_state(), apply(), get_current_snapshot(), begin_receiving_snapshot(), install_snapshot()
• RaftSnapshotBuilder: build_snapshot()

Added internal StorageIOResult trait for error conversion:

• Provides 14 methods like .sto_read_logs(), .sto_write_snapshot(), etc.

• Converts io::Error to StorageError<C> with proper context at Openraft boundaries

• Fix: Simplify storage trait error API: Accept io::Error from applications, manually convert to StorageError<C> with context #1457

Upgrade tip:

Storage implementors should update trait methods to return io::Error:

// Before
async fn read_vote(&mut self) -> Result<Option<Vote>, StorageError<C>> {
    self.vote.read().map_err(|e| StorageError::read_vote(&e))
}

// After
async fn read_vote(&mut self) -> Result<Option<Vote>, io::Error> {
    self.vote.read()
}

Return io::Error directly - Openraft adds context automatically.
The method body may leave unchanged: io::Error can be converted to
StorageError with a simple ?.

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This change is 

[xp-trumpet]

@xp-trumpet reviewed 17 of 26 files at r1, 2 of 2 files at r2, 1 of 1 files at r3, 11 of 11 files at r4, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @schreter)