fix: save downloaded ads with unresolved shipping options

[Tomblarom]

ℹ️ Description

download --ads=all currently skips direct-purchase-eligible ads when their displayed shipping price cannot be mapped to a predefined shipping option. Calculating the initial content hash raises sell_directly requires at least one predefined shipping_options entry.

Save those ads with explicit sell_directly: false and shipping_options: [], retain the extracted shipping costs, and warn the user to review shipping options before re-enabling direct purchase. Do not guess a carrier or inherit unrelated default options. Normal model validation remains unchanged.

Related context: #1256 addressed a different shipping extraction failure; this case has shipping_type: SHIPPING but no usable option. Reproduced with offline extraction tests against current main / latest release source.

📋 Changes Summary

• Normalize the unsupported direct-purchase combination during extraction, before initial hash validation.
• Add English/German warning and document the fallback.
• Cover absent/empty options, valid options, disabled direct purchase, retained costs/hash, and defaults that must not leak into the fallback.
• No dependencies, config defaults, or schema changes.

⚙️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (adds new functionality without breaking existing usage)
• 💥 Breaking change (changes that might break existing user setups, scripts, or configurations)

✅ Checklist

• I have reviewed my changes to ensure they meet the project's standards.
• I have tested my changes and ensured that all tests pass (pdm run test: 1,947 passed, 4 skipped; Xvfb, one worker).
• I have formatted the code (pdm run format).
• I have verified that linting passes (pdm run lint).
• I have updated documentation where necessary.

Tests never contact Kleinanzeigen. An earlier concurrent local run had two Node subprocess timeouts; the isolated affected suite (80 tests) and the subsequent complete run both passed.

Manual runtime validation after deployment: both previously failing ads downloaded successfully using the persisted account session (4 and 2 images). Both exported with the documented warning, sell_directly: false, and empty shipping options; no download errors were logged. Account data and ad content are intentionally omitted.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.16%. Comparing base (bfc646f) to head (d08a393).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1346   +/-   ##
=======================================
  Coverage   92.15%   92.16%           
=======================================
  Files          50       50           
  Lines        7804     7808    +4     
  Branches     1360     1361    +1     
=======================================
+ Hits         7192     7196    +4     
  Misses        401      401           
  Partials      211      211           

Flag	Coverage Δ
integration-tests	26.28% <0.00%> (-0.02%)	⬇️
smoke-tests	27.36% <0.00%> (-0.02%)	⬇️
unit-tests	91.07% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/kleinanzeigen_bot/extract.py	95.94% <100.00%> (+0.01%)	⬆️

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