fix: embed contract metadata and report compile-time version

[EurekaZainMalik]

Overview

This PR embeds contract metadata (contractmeta!) into the milestone-escrow WASM so explorers and stellar contract info meta can surface the contract name, version and source repo, and changes version() to return a compile-time CONTRACT_VERSION constant instead of an incrementing upgrade counter. DataKey::Version is retained as the storage schema version for migrations.

Related Issue

Changes

📦 Contract Metadata

• [MODIFY] contracts/milestone-escrow/src/lib.rs

  • Added contractmeta!(key = "name", val = "milesto-milestone-escrow").
  • Added contractmeta!(key = "version", val = env!("CARGO_PKG_VERSION")).
  • Added contractmeta!(key = "source_repo", val = "...") pointing at the source repository.
  • Introduced a compile-time CONTRACT_VERSION constant derived from CARGO_PKG_VERSION.
  • Changed version() to return CONTRACT_VERSION instead of reading DataKey::Version.
  • Kept DataKey::Version as the storage schema version used by migrations; initialize/upgrade no longer drive the reported version.

• [MODIFY] contracts/milestone-escrow/Cargo.toml

  • Ensured the crate version is the single source of truth consumed by env!("CARGO_PKG_VERSION").

• [ADD] contracts/milestone-escrow/src/version_tests.rs

  • Asserts version() matches the Cargo.toml package version.
  • Asserts version() is stable across upgrade calls (no longer increments).

Verification Results

cargo test -p milestone-escrow version_tests
✅ version() matches CARGO_PKG_VERSION
✅ version() unchanged after upgrade

Acceptance Criteria	Status
stellar contract info meta --wasm ... shows name, version and repo	✅ contractmeta! entries added for name, version and source repo
version() matches Cargo.toml in tests	✅ Covered by version_tests.rs

Closes #735

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