Add eco-tests for Mentat

[yeuyeuh]

Description

Adds eco-tests for the Mentat indexer, as recommended by the foundation for infrastructure that relies on protocol storage reads. These tests lock the signatures of the storage items, extrinsics, and runtime APIs that Mentat depends on, so that any type change, rename, removed item, or altered argument list on the protocol side breaks compilation in CI — surfacing the change before it ships rather than after it hits production.

Related Issue(s)

N/A

Type of Change

• Other (please describe): Adds CI eco-tests protecting external indexer infrastructure against breaking storage/API changes.

Breaking Change

None. This PR only adds tests under eco-tests/ and does not modify runtime or pallet code.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run ./scripts/fix_rust.sh to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• [] My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• [] New and existing unit tests pass locally with my changes
• [] Any dependent changes have been merged and published in downstream modules

Additional Notes

Submitted in response to the foundation's call for builders to add eco-tests. This is the first Mentat indexer eco-tests.