feat(backend): expose backend capabilities

[wolf31o2]

Summary

• add an optional, backward-compatible backend capability reporting API
• return typed unsupported errors for unavailable provider operations
• reject unsupported UTxO RPC additional UTxOs instead of silently ignoring them

Validation

• go test -race ./...
• go test -race ./backend/...

[Copilot]

Pull request overview

This PR introduces an optional capability-reporting layer for backend.ChainContext implementations, allowing callers to detect which operations a backend truly supports while preserving source compatibility for existing third-party contexts. It also standardizes “unsupported operation” failures via a typed ErrUnsupported / UnsupportedError, and tightens UTxO RPC evaluation behavior by rejecting unsupported additionalUtxos.

Changes:

• Add Capability, CapabilitySet, CapabilityReporter, and typed ErrUnsupported/UnsupportedError to the backend interface layer.
• Implement Capabilities() and return ErrUnsupported for unavailable operations across multiple backends (and wrappers).
• Add/extend tests to validate capability reporting and unsupported-operation error identity.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
backend/base.go	Introduces capability types/reporting helpers and the typed unsupported error used by backends.
backend/capabilities_test.go	Adds unit coverage for CapabilitySet.Has, legacy default behavior, and ErrUnsupported identity.
backend/blockfrost/blockfrost.go	Adds capability reporting (not advertising general additional-UTxO evaluation support).
backend/blockfrost/blockfrost_test.go	Tests Blockfrost capability reporting expectations.
backend/maestro/maestro.go	Adds capability reporting and returns typed unsupported errors for unsupported operations.
backend/maestro/maestro_test.go	Verifies Maestro capability reporting and typed unsupported error for GenesisParams.
backend/ogmios/ogmios.go	Adds capability reporting that depends on whether Kupo is configured; returns typed unsupported errors when Kupo is missing.
backend/ogmios/ogmios_test.go	Verifies capability reporting and typed unsupported errors for Ogmios-without-Kupo.
backend/utxorpc/utxorpc.go	Adds capability reporting, converts unsupported operations to typed errors, and rejects non-empty additionalUtxos for EvaluateTx.
backend/utxorpc/utxorpc_test.go	Tests UTxO RPC capabilities and typed unsupported errors for unsupported operations.
backend/cache/cache.go	Adds Capabilities() passthrough so cached contexts preserve wrapped capability sets.
backend/cache/cache_test.go	Tests that cached contexts preserve capability reporting.
backend/fixed/fixed.go	Adds capability reporting and converts several operations to typed unsupported errors (including changing some previously-successful stub returns).
backend/fixed/fixed_test.go	Tests fixed context capability reporting and typed unsupported errors for unsupported operations.

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