cardano-testnet: re-enable RPC ReadGenesis initialFunds assertion

[carbolymer]

Description

Stacked on #6655.

Re-enables the initialFunds assertion in the ReadGenesis RPC test.

The assertion was temporarily disabled because cardano-rpc read only the legacy sgInitialFunds field, while cardano-cli create-testnet-data provisions wallets via sgExtraConfig.secInitialFunds, so the RPC response reported no funds for testnet-data networks.

Depends on the cardano-rpc fix on IntersectMBO/cardano-api#1305, consumed here via a temporary source-repository-package stanza in cabal.project.

DO NOT MERGE until cardano-rpc is released to CHaP with the fix and the SRP stanza is dropped.
This PR exists to prove CI goes green with the fix in place.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Running tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
  • cardano-node-chairman, cardano-submit-api and cardano-testnet instead need a
    changelog fragment in <package>/.changes/, because their CHANGELOG.md is generated
    from fragments at release time. Copy _TEMPLATE.yml from that directory, or run
    nix run github:input-output-hk/cardano-dev#herald -- new
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG node developers to do this
for you.