fix: increase x402 payment timeout to 5min and add 402 re-sign logic

AI inference (especially DeepSeek) often exceeds the default 120s HTTP
timeout, causing the client to disconnect while the server completes
successfully — resulting in repeated payments on each retry.

Changes:
- Set X402Timeout = 5min for all x402 providers (Claw402, BlockRunBase, BlockRunSol)
- Handle 402 during payment retry by re-extracting Payment-Required
  header and re-signing instead of failing immediately
- Increase payment retry attempts from 3 to 5 for unstable gateways

mcp/payment/blockrun_base.go

@@ -72,6 +72,7 @@ func NewBlockRunBaseClientWithOptions(opts ...mcp.ClientOption) mcp.AIClient {
 		mcp.WithProvider(mcp.ProviderBlockRunBase),
 		mcp.WithModel(DefaultBlockRunModel),
 		mcp.WithBaseURL(DefaultBlockRunBaseURL),
+		mcp.WithTimeout(X402Timeout),
 	}
 	allOpts := append(baseOpts, opts...)
 	baseClient := mcp.NewClient(allOpts...).(*mcp.Client)