chore: pin soroban-sdk to exact version

[AugistineCreates]

This pr closes #503

This PR pins the core soroban-sdk workspace dependency to an exact minor/patch version to prevent untracked semantic/feature drift between local development, staging environments, and CI.

Key Changes

1. Workspace Dependency Pinning
   • Modified contracts/Cargo.toml to strictly pin soroban-sdk to =23.0.1 under [workspace.dependencies].
2. Workspace Cargo.toml Synchronization
   • Updated contracts/accountability_vault/Cargo.toml to utilize { workspace = true } for both soroban-sdk and soroban-sdk dev-dependencies, ensuring complete workspace-wide dependency alignment.
3. Lockfile Refresh
   • Ran cargo update inside the contracts directory to update contracts/Cargo.lock, resolving and strictly locking the workspace dependencies to exactly 23.0.1.
4. Upgrade & Dependency Pinning Policy
   • Added a new ## Upgrade & Dependency Pinning Policy section in contracts/README.md containing strict guidelines for future dependency bumps.

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