[core] Add eager failure for fallback branch read failures

[ArnavBalyan]

Purpose

• When reading from a fallback branch fails, the exception is swallowed and the reader falls through to the current branch. This can mask data issues, and the LOG.error does not include the exception object so the stack trace was lost.
• Added scan.fallback-branch.read-fail-fast (default false). When enabled the exception is propagated to the caller so the read fails immediately. When disabled the failure is logged with the full stack trace and the reader still falls through to the current branch.
• Fixes: [Bug] Fallback branch read silently swallows exceptions with no fail-fast option #7761

Tests

• UT

[ArnavBalyan]

cc @JingsongLi can you pls take a look thanks!