Add class-level Javadoc to CustomCheckoutClient, SubscriptionClient, and BaseClient

[omghante]

Problem

• StandardCheckoutClient has a class-level Javadoc comment:
  /** The StandardCheckout client class provides methods for interacting with the PhonePe APIs. */
• CustomCheckoutClient, SubscriptionClient, and BaseClient have no class-level Javadoc
• These are public API classes that merchants import and use directly
• Without docs, IDE tooltips show nothing — merchants can't tell which client to use
• Generated API documentation is incomplete

Proposed Changes

• Add class-level Javadoc to:
  • BaseClient — explain its role as the shared abstract base that handles OAuth2 token management, authenticated HTTP requests, and event telemetry
  • CustomCheckoutClient — explain it's for custom checkout flows with PCI-compliant card/token routing and device OS header injection
  • SubscriptionClient — explain it's for managing recurring payment mandates (setup, notify, redeem, cancel, status)
• Include @see cross-references between the client classes so merchants can navigate between them
• Keep the existing StandardCheckoutClient Javadoc unchanged