perf(file): read only header row instead of entire CSV in has_valid_headers

[vishali-mp]

Description

Optimized FileOutput.has_valid_headers() to read only the CSV header row instead of parsing the entire file into memory.

Before: csv.DictReader read all rows into dicts (list(csv_reader)) just to extract column names from the first row's keys.
After: csv.reader reads just the first line via next(reader) and compares sorted headers directly — no unnecessary I/O or memory allocation for data rows.

Also optimized the append path in out() to use pd.DataFrame.to_csv(mode="a") instead of reading the full CSV, concatenating, and rewriting.

Related Issue

#1220

Motivation and Context

On large CSV files, has_valid_headers() was O(n) in both time and memory for every out() call. This made append-mode writes progressively slower as the file grew. The fix makes header validation O(1) and append-mode writes O(1) per row.

How Has This Been Tested?

All 22 existing tests in tests/output_methods/test_file.py pass, along with the full test suite (420 passed, 2 skipped).

Screenshots (if appropriate):

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[SaboniAmine]

Thanks for your contribution! LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.17%. Comparing base (abfdd8c) to head (9114cdc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1235      +/-   ##
==========================================
- Coverage   89.18%   89.17%   -0.01%     
==========================================
  Files          47       47              
  Lines        4511     4510       -1     
==========================================
- Hits         4023     4022       -1     
  Misses        488      488              

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