test: implement comprehensive test suites for notification hub, env validator, testnet faucet, and countdown timer

[lolandriley-wq]

Summary

This pull request implements comprehensive unit test suites for four key SDK features:

• NotificationHub (Implement invoice payment notification pub/sub hub (in-process) #467): Pub/sub system for invoice payment events with typed channels, filtered subscriptions, and synchronous delivery
• EnvValidator (Build SDK environment variable validation at startup with helpful errors #466): Environment variable validation at SDK initialization with helpful remediation hints and secret redaction
• TestnetFaucet (Add Stellar testnet faucet integration for SDK dev mode #465): Testnet funding helper for integration tests with Friendbot retry logic and account polling
• InvoiceCountdownTimer (Build invoice deadline countdown precision timer with drift correction #464): Countdown timer with drift correction via ledger-time synchronization and visibility-aware syncing

Test Coverage

Each test suite includes:

• ✅ Basic functionality tests
• ✅ Error handling and edge cases
• ✅ Type safety and discriminated unions
• ✅ Filtering and subscription management
• ✅ Retry logic and rate limit handling
• ✅ Background tab detection and syncing

Fixes

Closes #467
Closes #466
Closes #465
Closes #464

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