Ercot product response handling

[kvkenyon]

Restore support for HAL and list response formats in _products_to_dataframe.

The _products_to_dataframe method was simplified and stopped handling HAL (_embedded.products, nested additional_properties._embedded.products) and raw list responses, causing get_list_for_products() to return empty DataFrames for valid API responses. This PR reintroduces logic to correctly parse these previously supported formats.

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Note

Reintroduces support for multiple products response shapes and strengthens test coverage.

• Updates ERCOT._products_to_dataframe to accept Any and correctly handle: {"products": [...]}, HAL {"_embedded": {"products": [...]}}, nested {"additional_properties": {"_embedded": {"products": [...]}}, and raw list [...]; returns empty DataFrame otherwise
• Adds regression tests validating DataFrame parsing for the above shapes
• Expands HTTP tests for products and version endpoints: correct URL paths, query handling, URL encoding, and basic error responses

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[cursor]

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