ci(contracts): add contracts-only Soroban build/test workflow

[samieazubike]

Closes #112

Summary

This PR adds a dedicated GitHub Actions workflow for Soroban contracts and scopes it to contract changes only.

What changed

• Added .github/workflows/contracts.yml with:
  • trigger on push and pull_request to main with paths: ['contracts/**']
  • Rust setup via dtolnay/rust-toolchain@stable targeting wasm32-unknown-unknown
  • dependency caching via Swatinem/rust-cache@v2
  • contract build step:
    • cargo build --target wasm32-unknown-unknown --release
  • contract test step:
    • cargo test
  • wasm artifact size logging step for target/wasm32-unknown-unknown/release/*.wasm

Security / best-practice improvements

• minimal workflow permissions: contents: read
• concurrency control to cancel superseded runs
• job timeout (timeout-minutes: 20)
• hardened bash in size-check step (set -euo pipefail, nullglob handling)

Cleanup

• Removed old .github/workflows/soroban_tests.yml to avoid overlapping/duplicate contract CI execution and ensure contracts CI is scoped to contracts/**.

Acceptance Criteria Mapping

• Contract workflow only triggers on changes to contracts/**
• Contracts build is verified for wasm32-unknown-unknown
• cargo test is executed in CI
• WASM output sizes are logged

Validation notes

• Workflow file structure and triggers verified locally by inspection.
• Local contract compilation currently reports pre-existing Rust contract errors (duplicate type definitions in subscription_renewal), which are outside the CI workflow implementation scope.

[drips-wave]

@samieazubike Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits