CI: add contract WASM size check to prevent exceeding Soroban limits

[Ejirowebfi]

Description

Soroban contracts have a maximum WASM size limit. There is no CI check to catch size regressions before deployment. A bloated contract will fail to deploy silently.

Tasks

• Add a step in .github/workflows/ci.yml that builds the optimized WASM
• Assert the file size is below 64 KB (current Soroban limit)
• Fail the CI job with a descriptive message if the limit is exceeded

Acceptance Criteria

• CI fails with a clear message when the WASM exceeds the size limit
• CI passes for the current optimized WASM
• The size threshold is configurable via a workflow input