docs: codify paykit issuer interoperability

[ovitrif]

Parent: #1207
Source: Pubky marketplace Bitkit team brief
Counterpart: synonymdev/bitkit-ios#713

Scope

• Mirror the issuer contract documented for iOS.
• Add focused tests for request filtering and JSON endpoint parsing in the Android implementation.

Acceptance criteria

• Android accepts and rejects the same fixtures as iOS for each network.