fix(backend): validate additional UTxOs

[wolf31o2]

Summary

• validate transaction input and output before additional-UTxO conversion
• reject typed-nil interface values without panicking
• add Blockfrost and Ogmios regression coverage

Testing

• go test -v -race ./backend/blockfrost ./backend/ogmios ./backend

[Copilot]

Pull request overview

This PR hardens backend transaction evaluation by validating “additional UTxOs” before they’re converted into backend-specific wire formats, explicitly rejecting both nil and typed-nil interface values (for TransactionInput / TransactionOutput) to avoid panics and confusing downstream failures.

Changes:

• Added backend.ValidateAdditionalUtxo (and a typed-nil-aware helper) to ensure additional UTxOs have both input and output populated.
• Enforced additional-UTxO validation in Ogmios conversion (commonUtxoToShared) and Blockfrost evaluation/request building.
• Added regression tests for both Ogmios and Blockfrost to confirm malformed additional UTxOs are rejected early (and before network requests for Blockfrost).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
backend/base.go	Adds shared validation helpers to reject missing/typed-nil UTxO fields.
backend/blockfrost/blockfrost.go	Validates additional UTxOs before evaluation and before building /evaluate/utxos request items.
backend/blockfrost/blockfrost_test.go	Adds regression coverage for rejecting malformed additional UTxOs and ensuring no HTTP request is made.
backend/ogmios/ogmios.go	Validates resolved UTxOs before converting them to Ogmios shared wire format.
backend/ogmios/ogmios_test.go	Adds regression coverage for rejecting malformed additional UTxOs (including typed-nil cases).

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