feat: add metadata field and accessor to commitment

[DeePrincipal-dev-lang]

feat: add metadata field and accessor to commitment

Summary

Adds an optional Map<String, String> metadata field to the Commitment struct so
off-chain context (e.g. risk notes, tags) passed by createCommitmentOnChain
round-trips through the contract.

Changes

• contracts/escrow/src/lib.rs

  • Imported Map from soroban_sdk
  • Added metadata: Map<String, String> field to Commitment struct
  • Added metadata parameter to create_commitment (accepted and stored as-is)
  • Added get_metadata(commitment_id) read accessor

• contracts/escrow/src/test.rs

  • Updated all existing create_commitment call sites to pass an empty Map
  • Added map import and Map to test imports

Remaining (in-progress)

• Metadata round-trip tests (get_metadata store/read, empty map, multi-key)
• contracts/README.md documentation update

Testing

Existing tests compile and pass with the updated signature. New
metadata-specific tests are pending.
closes #484

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