fix(server): warn and skip non-numeric filenames in consumer offset directories instead of panicking

[atharvalade]

Which issue does this PR close?

Closes #3132

Rationale

An operator accidentally placing a file (e.g. .DS_Store, editor swap file, backup) in the consumer offsets directory crashes the server on startup due to a panic! on non-numeric filenames.

What changed?

Both load_consumer_offsets and load_consumer_group_offsets in core/server/src/streaming/partitions/storage.rs panicked when encountering files with non-numeric names during startup offset loading.

The fix replaces both panic! calls with warn! log messages and continue, so the server gracefully skips unexpected files. Added 9 unit tests covering valid files, non-numeric files (.DS_Store, backup.bak), empty directories, subdirectories, and nonexistent paths for both functions.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. Minimal AI used
3. All tests verified locally with cargo test, cargo clippy, cargo fmt --check, and CI scripts
4. Yes, all code can be explained

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.16%. Comparing base (f5350d9) to head (4fc2476).

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3135       +/-   ##
=============================================
- Coverage     72.76%   55.16%   -17.60%     
  Complexity      943      943               
=============================================
  Files          1117     1115        -2     
  Lines         96368    86104    -10264     
  Branches      73544    63295    -10249     
=============================================
- Hits          70119    47503    -22616     
- Misses        23702    36043    +12341     
- Partials       2547     2558       +11     

Components	Coverage Δ
Rust Core	49.36% <100.00%> (-24.13%)	⬇️
Java SDK	62.30% <ø> (ø)
C# SDK	69.11% <ø> (-0.31%)	⬇️
Python SDK	81.43% <ø> (ø)
Node SDK	91.44% <ø> (-0.10%)	⬇️
Go SDK	39.41% <ø> (ø)

Files with missing lines	Coverage Δ
core/server/src/streaming/partitions/storage.rs	71.50% <100.00%> (+4.83%)	⬆️

... and 233 files with indirect coverage changes

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