fix: resolve silent NaN load data when fixed_year differs from snapshot year

[stan-buren]

Fixes #2187. When fixed_year differs from the snapshot year, load data was silently filled with NaN due to reindexing before year mapping.

• Reverse mapping direction: map snapshots to fixed_year first, then index into load data (approach by @gianvicolux)
• Add leap year validation guard (analysis by @luciboon)
• Add regression tests covering normal, leap year, and edge cases
• Move assert after reindexing to catch mapping failures

Closes #2187.

Changes proposed in this Pull Request

This PR resolves a critical issue where setting load.fixed_year different from the snapshots' year results in a DataFrame containing only NaN values. The problem stems from performing .reindex(index=snapshots) on load data that has not yet had its year replaced.

To address this, we implemented the following changes:

1. Added import calendar at the top of the scripts/build_electricity_demand.py file to enable leap year verification.
2. Reversed mapping order: Instead of slicing the year and reindexing immediately, we now map the snapshots' timestamps to the target fixed_year using snapshots.map(lambda t: t.replace(year=fixed_year)) first, index into the loaded dataset using this new index, and then restore the original snapshots index on the result.
3. Leap year guard: Added a check that raises a descriptive ValueError if the snapshots contain February 29th but the target fixed_year is not a leap year, prompting the user to drop the leap day or select a leap year.
4. Moved asset checks: Relocated the isna() assert statement to the end of the script (after reindexing) to catch any potential alignment failures.
5. Added Regression Tests: Created test/test_build_electricity_demand.py to cover same-year mappings, different-year mappings (regression test for Silent NaN load data when using fixed_year with snapshots from a different year #2187), leap-to-non-leap mappings, non-leap-to-leap error raising, default behavior when no fixed_year is set, and correctness of mapped values.
6. Added Release Notes: Documented this bugfix under the "Upcoming Release" section in doc/release_notes.md.

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.md.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.md files.

[stan-buren]

I've refactored the fixed year reindexing logic into a standalone helper function reindex_load_fixed_year in build_electricity_demand.py and imported it directly in test_build_electricity_demand.py.

This removes the code duplication in the test suite and ensures we are testing the actual production code.