feat: guard CMORisation against missing input files

[rbeucher]

Summary

Add a filename-based completeness check to file_discovery so that missing
monthly (or yearly) raw output files are caught before any file I/O or
CMORisation begins.

Changes

src/access_moppy/file_discovery.py

• _extract_year_month_from_path(path) — extends the year parser to also return
  the month, covering all known ACCESS filename conventions.
• MissingInputFilesError(FileDiscoveryError) — new exception with a .missing
  list of (year, month) tuples.
• check_file_completeness(paths, freq) — public, filename-only function that
  returns missing monthly or annual slots. Works for all sub-annual frequencies
  (mon, day, 3hr, 6hr, 1hr) and falls back to annual for yr / ocean files.
• discover_files(..., check_completeness=False) — when True, raises
  MissingInputFilesError before any file I/O. fx fields are exempt.

tests/unit/test_file_discovery.py

• 20 new unit tests covering all new code paths.

Usage

files = discover_files(
    archive_root, "6hrPlevPt.hurs",
    model_id="ACCESS-ESM1-6",
    check_completeness=True,   # raises MissingInputFilesError if months absent
)

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.5%. Comparing base (f14320a) to head (2a7c80b).

Files with missing lines	Patch %	Lines
src/access_moppy/file_discovery.py	90.0%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #554     +/-   ##
=======================================
+ Coverage   74.3%   74.5%   +0.1%     
=======================================
  Files         36      36             
  Lines       7274    7343     +69     
  Branches    1354    1373     +19     
=======================================
+ Hits        5408    5470     +62     
- Misses      1581    1584      +3     
- Partials     285     289      +4     

Flag	Coverage Δ
unit	74.5% <90.0%> (+0.1%)	⬆️

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