fix(manual-api): allow multiple overrides for lists (rates_import_override, rates_export_override)

[chickenonaraft88]

What

Fixes #4405

Allow non-indexed dict_list overrides to accumulate rather than assuming they are all at index 0.

Fix

Both api_select_update() (storage) and get_arg() (the list-merge fetch.py's basic_rates() consumes) are now index-aware:

• A no-index dict_list override only collides with an exact duplicate resend - distinct windows coexist.
• An explicit index still replaces on resend, and distinct explicit indices still coexist, preserving the original indexed-override behaviour.
• On read, a dict_list override's stored index only ever matters for write-time dedup - it has no meaning as an output list position, since these overrides are a bag of independent entries, not per-slot values.

Test plan

test_manual_api.py gained coverage for the storage-layer dedup rules, the get_arg/basic_rates merge behaviour, and the malformed-index guard - no prior test exercised multiple simultaneous no-index overrides at all, which is how this went unnoticed.

• ./run_all --test manual_api --test basic_rates --test validate_config --test annual_tariff passes
• ./run_pre_commit - all hooks pass (the one pre-existing failure, test_faq_recorder_config, is an unrelated Windows-only cp1252 decoding bug reproducible on main, not caused by this change)

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