fix: cleanup_stream removes orphaned StreamMetadata and Delegate entries

[samsolo247]

Summary

Fixes #220.

cleanup_stream removed Stream(id) and its four index entries, but left StreamMetadata(id) and Delegate(id) as orphaned persistent ledger entries — growing unbounded over the contract's lifetime.

Changes

contracts/streaming/src/lib.rs

Extended cleanup_stream to remove DataKey::StreamMetadata(id) and DataKey::Delegate(id) after deleting the stream entry. A .has() guard is used before each removal because both fields are optional — calling .remove() on an absent key panics in Soroban.

// Remove optional associated entries if they exist.
if env.storage().persistent().has(&DataKey::StreamMetadata(stream_id)) {
    env.storage().persistent().remove(&DataKey::StreamMetadata(stream_id));
}
if env.storage().persistent().has(&DataKey::Delegate(stream_id)) {
    env.storage().persistent().remove(&DataKey::Delegate(stream_id));
}

contracts/streaming/src/test.rs

Two new tests:

• test_cleanup_stream_removes_all_associated_storage_keys — creates a stream, sets both metadata and a delegate, cancels the stream, calls cleanup_stream, then asserts all three keys (Stream, StreamMetadata, Delegate) are absent.
• test_cleanup_stream_works_without_optional_entries — verifies cleanup_stream does not panic when neither metadata nor delegate were ever set.

Testing

cargo test

All 123 tests pass (121 pre-existing + 2 new).

Closes #220