fix(receipt): preserve subscriptionId metadata

[EfeDurmaz16]

What

• Add optional subscription_id support to Mpp::Receipt.
• Preserve subscriptionId when parsing and serializing Payment-Receipt.
• Add regression coverage for foreign receipts and round trips.

Why

Subscription activation and renewal receipts can carry a server-issued subscriptionId. Ruby currently drops this field during parsing, while the Python, Go, and Rust SDKs preserve it.

How

Map Ruby's optional subscription_id field to the wire-level subscriptionId field. Existing receipts remain compatible, and this PR does not implement the subscription intent itself.

Verification

• bundle exec rake test: 336 tests, 836 assertions
• bundle exec standardrb
• bundle exec srb tc

Not included

• Subscription activation or renewal
• Access-key authorization
• Subscription stores or framework APIs