feat: Added key method of ListElement

[tbkr]

Description

For a ListElement we are now able to query the keys. The keys of the ListElement are returned as a dict[str, str] and resemble the MARS keys on the individual level of the Schema.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/fdb/pull-requests/PR-257

[Copilot]

Pull request overview

This PR adds a way to query the (combined) MARS keys from a ListElement in the Python API, exposing them as a dict[str, str] via the pybind layer and the Python wrapper.

Changes:

• Expose fdb5::ListElement::combinedKey() as a Python-accessible key() returning a dict[str, str] (via std::map).
• Add a ListElement.key property (and expanded docstrings) in the Python wrapper.
• Add an integration test exercising the new key attribute across list levels.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
tests/pyfdb/integration/test_list.py	Adds a parametrized integration test validating ListElement.key behavior per schema level.
src/pyfdb_bindings/bindings.cc	Adds a ListElement.key binding that returns the combined key as a Python dict.
src/pyfdb/pyfdb_iterator.py	Adds ListElement.key property + docstrings; also changes offset/length API shape/behavior.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.19%. Comparing base (ef36e0f) to head (def1c69).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #257      +/-   ##
===========================================
- Coverage    71.22%   71.19%   -0.03%     
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  Files          376      376              
  Lines        23762    23762              
  Branches      2478     2478              
===========================================
- Hits         16924    16918       -6     
- Misses        6838     6844       +6     

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[danovaro]

lgtm, only a small remark on the documentation

[simondsmart]

I would like to have a mechanism to extract either/both the 3 separated keys, correspnding to the the levels of the FDB, or the combined key. This only gives the combined key.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

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