[Econ] Port bedding cost preprocessing to a special-case handler

[matthew7838]

Context

Issue(s) closed by this pull request: closes #3088
Ports the implementation from #3117 (bedding-mapping) onto the refactor_preprocess special-case architecture; #3117 remains the reference for the review history.

What

Implements the Bedding requirements line item as a special-case handler (RUFAS/EEE/economics/special_cases/bedding_requirements.py, registered as BeddingRequirementsHandler) on top of the preprocessing refactor, replacing the dedicated_processor dispatch used on the bedding-mapping branch. Also fixes the base branch's dangling SeedCostHandler import (special_cases/seed_costs.py does not exist on refactor_preprocess, so the package did not import).

Why

The generic pipeline computed bedding cost as one blended average head count times one blended price, and the pen bedding_name (a user config name such as calf_straw) never matched the price-file keys (straw), so every pen fell back to a $1/head placeholder. On the example freestall farm this produced ~$42.50/yr instead of the correct value. The refactor branch introduces the special-case handler architecture, and this line item belongs there.

How

• BeddingRequirementsHandler.process() resolves each pen bedding_name -> bedding_configs.bedding_type -> price file (matching pens by their id field, not list position), then bills (average head present that year) x (that year's dollar-per-head price) per calendar year, summed across pens and years.
• Only pens whose animal combination is in the mapping's billable_pen_combinations (["LAC_COW"]) are billed - the SME-confirmed price basis is per lactating cow. Pens with bedding_type "none" incur no cost.
• bedding_type_to_file_key in mapping.py is the complete registry of billable bedding types (identity entries deliberate); lookups are strict.
• Missing price years fall back to the nearest available year with a warning (the bedding CSVs currently carry only a 2021 column); missing FIPS rows, unloadable price files, malformed bedding configs, and non-numeric output values all warn instead of failing silently.
• biophysical_aggregate reports the billed quantity (head-years) so quantity x price_aggregate reconciles with the line-item total; headline results are also emitted as econ_bedding_total_cost, econ_bedding_billed_head_years, and econ_bedding_avg_price_per_head_year for lightweight report filters.
• The handler reads its configuration from the Bedding requirements entry in ECONOMIC_MAP, keeping the mapping the single declarative registry.

Test plan

• 27 tests pass in tests/test_EEE/test_economics_preprocessing.py (14 ported bedding tests covering per-pen pairing, name-to-type resolution, compound-type normalization, none-skip, leap/partial-year units, multi-year per-year pricing, out-of-order pen ids, missing price file, missing FIPS, quantity-x-price reconciliation, and the framework breakdown flow).
• Full 7-year freestall example run end-to-end on this branch: bedding line item is 623.02 head-years x $53.22 = $33,156.91, an exact match with the bedding-mapping branch output; quantity x price reconciles to the cent.
• Black-clean; mypy reports no issues in the new handler module.

Input Changes

• input/data/EEE/economics_map.json - Bedding requirements block updated to mirror mapping.py (reference file, not loaded at runtime). No user-facing input schema change; animal.bedding_configs and animal.pen_information already exist.

Output Changes

• BeddingRequirementsHandler.process.econ_bedding_total_cost: total bedding cost in dollars.
• BeddingRequirementsHandler.process.econ_bedding_billed_head_years: billed quantity in head-years.
• BeddingRequirementsHandler.process.econ_bedding_avg_price_per_head_year: average dollar-per-head-per-year price used.
• The bedding line item within econ_preprocessed_economic_inputs / econ_economic_line_item_breakdown now reflects correct per-pen annual cost (values change materially vs the base branch).

Filter

{
    "name": "Bedding cost (issue #3088)",
    "filters": [
        "econ_bedding_total_cost$",
        "econ_bedding_billed_head_years$",
        "econ_bedding_avg_price_per_head_year$"
    ]
}