Big Endian: add daily workflow UT job and fix UTs

[rainsupreme]

Big endian support on Valkey is "best effort" and not guaranteed, but we haven't been doing any regular testing at all afaik. This PR adds a job to the daily workflow to run UTs on an emulated big endian platform. Integration tests failed excessively because of how slow emulation is.

I fixed several problems with tests and improved UT coverage of key points where endian byte order matters - and fwiw I didn't find any bugs. I think the main coverage gap remaining after this is RDB serialization (maybe little endian <-> big endian round trips?)

And why did I do this? I wrote a couple lines of endian-specific code for #3166 and wanted to test it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.97%. Comparing base (fc217fc) to head (4078448).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #3330      +/-   ##
============================================
- Coverage     74.98%   74.97%   -0.01%     
============================================
  Files           129      129              
  Lines         71553    71553              
============================================
- Hits          53652    53648       -4     
- Misses        17901    17905       +4     

Files with missing lines	Coverage Δ
src/server.h	100.00% <ø> (ø)

... and 29 files with indirect coverage changes

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[Scottcjn]

Great to see big-endian getting proper CI attention in Valkey!

We run an IBM POWER8 S824 (16-core, 128 threads, 512GB RAM) as a daily driver for LLM inference and blockchain attestation — native big-endian ppc64. Happy to run the full Valkey test suite on real hardware if that would be useful for validating beyond QEMU emulation.

We also have Power Mac G5s (PPC64 big-endian, Mac OS X / Linux) and PowerPC G4s (32-bit big-endian) if older architectures are of interest.

Real hardware catches timing-dependent and memory-ordering issues that emulation often misses. Let us know if we can help.

[madolson]

@Scottcjn That would be amazing if you could help run some tests! We at some point tried to acquire some IBM hardware for testing and hit some snags. In the past we've discussed that IBM is sort of tier 2 supported, we will accept fixes but since we can't reliably test on it it's hard to say it's officially supported.

[rainsupreme]

Here's a run of just the new job: https://github.com/rainsupreme/valkey/actions/runs/22814475783