ci: run Soroban contract tests in CI as a quality gate

[Horlarmmy]

What does this PR do?

Adds contract testing to CI as a required quality gate. Previously the
contracts job only ran cargo fmt --check, cargo clippy, and cargo build, so behavioral regressions could slip through to main.

The job (renamed Contracts Build & Test) now also runs the full Soroban
workspace test suite and fails the pull-request workflow on any failure.

Closes #886

Changes

• .github/workflows/ci.yml
  • Add a Test step running cargo test -- --nocapture
  • Set RUST_BACKTRACE=1 and CARGO_TERM_COLOR=always for clear,
    deterministic failure output
  • No continue-on-error / || true — test failures fail the job
  • Existing rustfmt, clippy, build, and Swatinem/rust-cache steps
    remain unchanged
• docs/governance/CONTRIBUTING.md
  • Add a "Local Verification" section so contributors can run the exact
    same commands before opening a PR

Local verification

cd contracts
cargo fmt --check
cargo clippy
cargo build
cargo test
Verified: cargo test -- --nocapture passes on a clean environment
(255 tests, 0 failures).
Acceptance criteria
- Contract test failures fail the pull-request workflow
- Command runs successfully on a clean GitHub-hosted runner
- Existing build, formatting, and Clippy checks remain intact
- CI logs identify the failing package and test (Cargo failures: output)
- Contribution docs contain the matching local verification command

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