Fix error message if PS connection is lost while receiving prefetch

[hlinnaka]

If the pageserver connection is lost while receiving the prefetch request, the prefetch queue is cleared. The error message prints the values from the prefetch slot, but because the slot was already cleared, they're all zeros:

LOG:  [NEON_SMGR] [shard 0] No response from reading prefetch entry 0: 0/0/0.0 block 0. This can be caused by a concurrent disconnect

To fix, make local copies of the values.

In the passing, also add a sanity check that if the receive() call succeeds, the prefetch slot is still intact.

[arpad-m]

context: https://neondb.slack.com/archives/C083A7FA0QY/p1732798568216509

[github-actions]

6941 tests run: 6633 passed, 0 failed, 308 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_read_validation: release-x86-64

Postgres 15

• test_pull_timeline[True]: release-arm64

Postgres 14

• test_compute_pageserver_connection_stress: release-x86-64

Code coverage* (full report)

• functions: 30.6% (7983 of 26064 functions)
• lines: 48.5% (63379 of 130562 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
cf7b289 at 2024-11-28T14:30:14.284Z :recycle:}