Parse Ogmios protocol ratios instead of evaluating them

[SashaMIT]

Summary

• OgmiosV6ChainContext._fetch_protocol_param passed pledge influence, monetary expansion, treasury expansion, and script prices through eval. A value of 1+1 was accepted as 2.
• Those fields now go through the existing _fraction_parser, which only accepts numerator/denominator. 1/10 is still 0.1, and 721/10000000 is still the memory price.

Test plan

• pytest test/pycardano/backend/test_ogmios_v6_ratio.py fails on tip (Failed: DID NOT RAISE ValueError for 1+1) and passes after the change (2 passed).

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