ExtractPaymentDetails always takes Accepts[0] — never matches the client's chain

[VickyXAI]

Problem

ExtractPaymentDetails unconditionally returns the first option, and its own doc comment says so:

// x402.go:243-250
// ExtractPaymentDetails extracts payment details from a PaymentRequirement.
// Returns the first payment option if multiple are available.
func ExtractPaymentDetails(req *PaymentRequirement) (*PaymentOption, error) {
	...
	option := req.Accepts[0]

createPaymentPayload then dispatches purely on the client's own chain and never reads option.Network:

// base_client.go:203-205
if bc.isSolana() {
	return CreateSolanaPaymentPayload(bc.solanaKey, option, resourceURL, description, extensions, bc.solanaRPCURL)
}

Nothing ever checks that the selected option is one this client can actually pay.

Impact

Multi-option 402s break. If a gateway offers [base, solana] in that order, a Solana client takes the Base option, hands an ERC-20 hex address to CreateSolanaPaymentPayload, and dies at invalid asset mint — even though a payable Solana option was sitting right there in Accepts[1].

The error message actively misleads. A Solana client receiving a Base-only 402 (mis-set BLOCKRUN_SOLANA_API_URL, a WithAPIURL override, or a multi-chain gateway) now fails with:

feePayer is required in payment requirement extra for Solana transactions

That blames the server for a client-side chain mismatch and sends users hunting a gateway bug that doesn't exist. Before #9 the same case stopped earlier at "no wallet is configured" — also wrong, but it at least pointed at the client.

Fix

1. Have ExtractPaymentDetails select the option matching the client's chain rather than always taking [0] (it needs the chain passed in, or the selection moves to createPaymentPayload).
2. When no option matches, fail explicitly:

return "", &PaymentError{Message: fmt.Sprintf(
    "client is configured for Solana but the 402 only offers %q", option.Network)}

Scope note: this is about selecting a payable option and reporting mismatches, not about capping spend — per-call spend limits are a settled no.

Provenance

Surfaced independently by Codex and the adversarial pass during /review of #9.