fix(header-serde): change train() to return Result and skip subdirectories

[kushdab]

Summary

Fixes #925.

The dict::train() function had two related TODOs:

1. Skip subdirectories fs::read_dir yields both files and directories. Passing a directory path to zstd::dict::from_files causes an IO error at runtime. The fix adds an is_file() guard so only regular files are included in the training set.

2. Return Result instead of panicking the two .unwrap() calls (one on read_dir, one on dict::from_files) caused a silent process-kill on any bad path or training failure. The return type is now Result<Vec<u8>> using the crate's existing pingora_error machinery (OrErr::explain_err).

Changes

pingora-header-serde/src/dict.rs

• Return type: Vec<u8> → Result<Vec<u8>>
• fs::read_dir(...).unwrap() → .explain_err(InternalError, ...)?
• Added .is_file() filter in the filter_map closure to skip subdirectories
• dict::from_files(...).unwrap() → .explain_err(InternalError, ...)
• Updated existing test helper from train(path) to train(path).expect(...)
• Added test_train_skips_subdirectories regression test

pingora-header-serde/src/trainer.rs

• main() now returns Result<(), Box<dyn std::error::Error>> and propagates errors with ?, so the binary exits with a non-zero code and a readable message instead of panicking silently