Feature: Add CI pipeline for contract build and test

[AJ0070]

Closes #81 .

This PR introduces a comprehensive GitHub Actions CI pipeline (ci.yml) to automate the building, testing, and validation of the Bridgelet Core smart contracts. It also includes corresponding updates to our helper scripts and testing documentation to reflect the new workspace-wide workflows.

Changes Included

• New CI Workflow (.github/workflows/ci.yml):
  • Lint Job: Runs cargo fmt and cargo clippy with warnings treated as errors.
  • Test Job: Executes unit and integration tests across all workspace members (bridgelet-shared, ephemeral_account, reserve_contract, sweep_controller).
  • Build Job: Compiles WASM artifacts for all contracts, runs stellar contract optimize, and uploads them as downloadable GitHub artifacts.
  • Sandbox Smoke Test Job: Spins up a local Stellar network using the stellar/quickstart:testing Docker image, deploys the built ephemeral_account.wasm, and invokes it to ensure end-to-end viability.
• Legacy Workflow Update (test.yml): Modernized to test the entire workspace (serves as a backwards-compatible shim for existing branch protection rules).
• Script Updates (scripts/build.sh, scripts/test.sh): Refactored to loop through and build/test all contracts in the workspace by default, mirroring the CI pipeline.
• Documentation (docs/testing.md): Added a new "CI Pipeline (GitHub Actions)" section documenting the jobs, caching strategy, and pinned toolchains.

Acceptance Criteria Met

• Rust implementation complete
• cargo test passing across the workspace
• Integration test on local Stellar sandbox passing via Docker smoke test
• Docs updated in docs/testing.md

[AJ0070]

@phertyameen Please approve the workflow.