Support multiple ERCOT products response shapes

[kvkenyon]

Note

Scope: Improve robustness of ERCOT products parsing and add regression coverage.

• Update ERCOT._products_to_dataframe to accept diverse response shapes: plain {"products": [...]}, HAL {"_embedded": {"products": [...]}}, nested {"additional_properties": {"_embedded": {"products": [...]}}}, and raw lists; handles None/non-dict inputs safely
• Add regression tests verifying DataFrame conversion for each shape and that get_product dispatches the correct URL

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[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tinygrid/ercot/client.py	90.62%	3 Missing ⚠️

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