Add cluster-persist-config option to control nodes.conf save behavior

[enjoy-binbin]

This commit introduces a new configuration option cluster-persist-config
that controls how the cluster handles nodes.conf file save failures.

The option supports two modes:

• sync (default): Synchronously save the config file. If the save fails,
  the process exits. This maintains backward compatibility with the old
  behavior (before 9.1).
• best-effort: Synchronously save the config file. If the save fails,
  only log a warning and continue running. This allows the node to survive
  disk failures (e.g., disk full, read-only filesystem) without exitting,
  giving administrators time to address the issue.

Note that this modifies the behavior of #1032, whereas #1032 was "best-effort",
we have now introduced a configuration option that defaults to "sync."
See #1032 discussion for more details.

Background:
When a disk becomes read-only or full, any cluster metadata change would
trigger a nodes.conf save attempt. With the old behavior, the node would
immediately exit via clusterSaveConfigOrDie(), potentially causing multiple
nodes on the same machine to crash simultaneously, leading to cluster
unavailability.

The new best-effort mode addresses this by allowing nodes to continue
operating even when disk writes fail. This is particularly useful in cloud
environments where disk failures are more common due to scale.

Note: Startup-time config saves (in clusterInit and verifyClusterConfigWithData)
still use clusterSaveConfigOrDie() since disk issues at startup should cause
immediate failure.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.50%. Comparing base (6c329df) to head (b7f97d5).
⚠️ Report is 3 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #3372      +/-   ##
============================================
- Coverage     74.55%   74.50%   -0.05%     
============================================
  Files           130      130              
  Lines         72731    72734       +3     
============================================
- Hits          54225    54192      -33     
- Misses        18506    18542      +36     

Files with missing lines	Coverage Δ
src/cluster_legacy.c	88.16% <100.00%> (-0.16%)	⬇️
src/config.c	77.70% <ø> (ø)
src/server.h	100.00% <ø> (ø)

... and 21 files with indirect coverage changes

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[zuiderkwast]

When we merge #2555 we can change 'best-effort' to be in bio?

[murphyjacob4]

Makes sense to me

We had quorum in Monday's meeting and got the verbal consensus. FYI @valkey-io/core-team in case anyone wasn't there and wants to chime in

[hpatro]

Shall we name it as cluster-config-save-behavior ?

[enjoy-binbin]

When we merge #2555 we can change 'best-effort' to be in bio?

Yes, we can, it is best-effort.

Shall we name it as cluster-config-save-behavior ?

Sound like a better (a more specific one) name. I am open to it.