feat(server): add threads count and disk space to sysinfo stats

[seokjin0414]

Summary

Closes #2732.

• Add threads_count, free_disk_space, total_disk_space fields to Stats struct
• Collect thread count via sysinfo Process::tasks() API (Linux only, 0 on macOS)
• Collect disk space via fs2 crate (statvfs syscall) with warning log on failure
• Add IggyUsage (messages_size_bytes), Disk (free/total), Threads (conditional) to sysinfo log output
• Update binary protocol encoding/decoding with backward-compatible field appending
• Add CLI Table/List output rows for new fields
• Add integration tests: field presence in all 4 output formats + JSON value verification with message size check

Design

• Thread count: process.tasks().map(|t| t.len()).unwrap_or(0) — uses existing refresh_processes() call, no additional sysinfo refresh needed. Conditional log output (skipped when 0, matching OpenFDs pattern)
• Disk space: fs2::available_space() / fs2::total_space() — single statvfs syscall per metric (~10μs), already used in logger.rs. Logs warning on failure instead of silently defaulting to 0
• IggyUsage: existing messages_size_bytes field (iggy metadata only, no sysinfo) — added to sysinfo log output
• Binary protocol: new fields appended after cache_metrics, decoded with current_position + N <= payload.len() guards for backward compatibility

[codecov]

Codecov Report

❌ Patch coverage is 74.50980% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.26%. Comparing base (6a6f155) to head (28d5840).

Files with missing lines	Patch %	Lines
core/binary_protocol/src/utils/mapper.rs	80.64%	3 Missing and 3 partials ⚠️
core/server/src/shard/system/stats.rs	45.45%	6 Missing ⚠️
...server/src/shard/tasks/periodic/sysinfo_printer.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2917      +/-   ##
============================================
- Coverage     70.27%   70.26%   -0.01%     
  Complexity      862      862              
============================================
  Files          1029     1029              
  Lines         85283    85332      +49     
  Branches      62653    62715      +62     
============================================
+ Hits          59932    59958      +26     
- Misses        22839    22850      +11     
- Partials       2512     2524      +12     

Flag	Coverage Δ
csharp	67.43% <ø> (-0.23%)	⬇️
go	36.36% <ø> (ø)
java	59.87% <ø> (ø)
node	91.37% <ø> (-0.07%)	⬇️
python	81.43% <ø> (ø)
rust	70.64% <74.50%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ore/binary_protocol/src/cli/binary_system/stats.rs	95.00% <ø> (ø)
core/common/src/types/stats/mod.rs	69.86% <100.00%> (+1.29%)	⬆️
core/server/src/binary/mapper.rs	94.41% <100.00%> (+0.04%)	⬆️
...server/src/shard/tasks/periodic/sysinfo_printer.rs	78.26% <66.66%> (-0.81%)	⬇️
core/binary_protocol/src/utils/mapper.rs	79.34% <80.64%> (+0.05%)	⬆️
core/server/src/shard/system/stats.rs	91.20% <45.45%> (-6.36%)	⬇️

... and 12 files with indirect coverage changes

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[hubcio]

in PR description you say Disks, yet in code I don't see this being used. also please fix my comment in stats.rs. other than that LGTM.

[hubcio]

please add warning print when it fails

[seokjin0414]

Thanks for the review~
Fixed both — added tracing::warn! on fs2 failure in stats.rs (28d5840) and updated PR description to reflect fs2 instead of Disks