Add testnet parameter documentation for getPaymentStatus

[spencerstock]

What changed? Why?

Added the testnet parameter to getPaymentStatus() function calls in the accept-payments documentation. This parameter must match the testnet setting used in the original pay() call to ensure payment status is checked on the correct network (mainnet vs testnet).

Added:

• testnet: true parameter to the client-side example
• Note explaining the importance of matching testnet parameters between pay() and getPaymentStatus() calls

How has it been tested?

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1