Update API docs for session-based billing - #110

Running Vale across on the following files affected by this PR:

- api-reference/gateway.mdx
- api-reference/wallet.mdx
- payments/mpp.mdx

Attempting to initialize Vale linter...
No customer vale configuration detected. Using Mintlify's default configuration.
Attempting to run Vale spellcheck...
Vale spellcheck completed successfully.

api-reference/gateway.mdx:
	- 8:205      Did you really mean 'blockchain'?
	- 214:99     Did you really mean 'respawning'?
	- 220:71     Did you really mean 'Agentbot'?
	- 226:133    Did you really mean 'loopback'?
	- 232:61     Did you really mean 'loopback'?
	- 233:50     Did you really mean 'Tailscale'?
	- 263:86     Did you really mean 'signup'?

api-reference/wallet.mdx:
	- 8:88      Did you really mean 'Coinbase'?
	- 20:79     Did you really mean 'blockchain'?
	- 45:4      Did you really mean 'pathUsd'?
	- 195:32    Did you really mean 'Agentic'?
	- 235:32    Did you really mean 'Coinbase'?
	- 283:11    Did you really mean 'viem'?
	- 283:38    Did you really mean 'Sepolia'?

payments/mpp.mdx:
	- 3:41      Did you really mean 'crypto'?
	- 3:97      Did you really mean 'blockchain'?
	- 8:94      Did you really mean 'blockchain'?
	- 24:9      Did you really mean 'Mainnet'?
	- 25:9      Did you really mean 'Testnet'?
	- 150:66    Did you really mean 'Agentbot'?
	- 152:50    Did you really mean 'testnet'?
	- 152:69    Did you really mean 'mainnet'?


Found 22 suggested changes across 3 files in 718.69ms