branch-4.0: [fix](cloud) fix segment footer CORRUPTION not triggering file cache retry

[Hastyshell]

Proposed changes

Backport of #61386 to branch-4.0.

The three-tier retry logic in Segment::_open() (static method) was structured as if-else-if, so when open_file() succeeded but _parse_footer() returned CORRUPTION, the retry branch was unreachable.

Root cause

// Before: if-else-if structure
auto st = fs->open_file(path, &file_reader, &reader_options);
if (st) {                                  // open_file succeeded (almost always)
    segment->_file_reader = ...;
    st = segment->_open(stats);            // _parse_footer() → CORRUPTION stored in st
} else if (st.is<CORRUPTION>() && ...) {   // UNREACHABLE: already entered `if` branch
    // Tier 1: clear cache, retry
    // Tier 2: bypass cache, read remote directly
}
RETURN_IF_ERROR(st);  // CORRUPTION returned without any retry

open_file() only opens a file handle and rarely returns CORRUPTION. The actual footer checksum validation happens inside _parse_footer() (called via segment->_open()). Because the retry was in an else if guarded by the same st from open_file(), it was never reachable for the common _parse_footer() corruption case.

Fix

Change else if to a separate if block, so CORRUPTION from either open_file() or _parse_footer() triggers the three-tier retry.

Issue

Observed in cloud (S3) deployments (branch-4.0): schema change fails with CORRUPTION: Bad segment file footer checksum not match. Log analysis confirmed that no retry log messages were ever emitted, consistent with this code bug.

Tests

• Added TestFooterCorruptionTriggersRetry to segment_corruption_test.cpp
• Uses the existing Segment::parse_footer:magic_number_corruption sync point to corrupt the footer magic number on the first _parse_footer() call only

Checklist

• Does it affect the original behavior? Yes — previously CORRUPTION from _parse_footer() was never retried; now it correctly follows the three-tier retry
• Has unit test been added? Yes
• Is this a backport? Yes, backport of [fix](cloud) fix segment footer CORRUPTION not triggering file cache retry #61386

[Thearas]

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