Add forward-compatibilty test and fix for using old client on new node

[MitchTurner]

Linked Issues/PRs

Description

We are addressing a problem with forward-compatibility in our fuel-core client. We have recently added new consensus parameter versions and if users haven't updated their client to support the new parameters, it is just going to error. We want to prevent this from happening with the following changes:

This PR does two things

1. It changes the way that the graphql server handles requests for consensus parameters. If you ask for params but don't specify any V2 parameters, then it will assume you are querying from a client that only supports V1 params and will convert V2 parameters to V1 before returning
2. Change the way the client works so if it receives unsupported consensus parameters in the future, it will emit and error and convert to the latest consensus parameter version optimistically.

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

After merging, notify other teams

[Add or remove entries as needed]

• Rust SDK
• Sway compiler
• Platform documentation (for out-of-organization contributors, the person merging the PR will do this)
• Someone else?

[cursor]

PR Summary

Medium Risk
Changes GraphQL responses and client-side decoding of consensus parameters to tolerate unknown/future versions by downgrading shapes/values, which can affect consensus-parameter interpretation across versions. Risk is mitigated by added unit/integration tests but touches core chain-parameter plumbing used by many clients.

Overview
Improves forward compatibility for consensusParameters across mismatched client/node versions.

On the GraphQL server, consensus_params_for_selection inspects the query Lookahead and, when V2-only fields aren’t requested, returns a legacy-compatible shape by downgrading ScriptParameters to V1 and GasCosts from V7 to a V6-compatible representation (filling legacy-only fields with defaults). This behavior is applied to both the chain query and the upgrades query.

On the client, decoding of consensus parameters is changed so Unknown (future) enum variants no longer hard-error; instead it logs a warning and optimistically degrades to the latest locally-supported conversion for TxParameters, PredicateParameters, ScriptParameters, ContractParameters, FeeParameters, GasCosts, and ConsensusParameters (including legacy fragments). Adds targeted tests for unknown-version degradation and server-side downgrade behavior, plus new version-compatibility snapshot config and dev-deps for the forkless-upgrade test harness.

^{Reviewed by Cursor Bugbot for commit 8b93a69. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 469766f. Configure here.}