docs: clarify TEE endpoint discovery and x402 payment settlement

[addnad]

Summary

Clarifies TEE endpoint discovery and x402 payment settlement in the LLM documentation to resolve confusion about which endpoint URL to use and prevent costly developer errors.

Problem

Issue #229 reported critical inconsistencies that created confusion and risk:

• Overview docs listed one endpoint URL (https://llm.opengradient.ai)
• Code examples used a different URL (https://llmogevm.opengradient.ai)
• Developers didn't know which URL was correct
• Using the wrong endpoint could result in failed transactions
• Real money ($OPG tokens) could be lost due to misconfiguration

Solution

Added a comprehensive section to docs/opengradient/client/llm.md explaining:

Production Use: Automatic Endpoint Discovery

• Default LLM() constructor automatically discovers active TEE endpoints from on-chain registry
• Recommended approach for production deployments
• Ensures high availability through automatic failover

Development Use: Manual Endpoint Configuration

• LLM.from_url() for hardcoded endpoints
• Perfect for self-hosted or testing scenarios
• TLS certificate verification disabled for self-signed certs

x402 Payment Settlement (Key Clarification)

• Regardless of which TEE endpoint serves the inference request, x402 payments always settle on Base Sepolia blockchain
• This ensures consistency, auditability, and independent payment tracking
• Developers can safely switch between endpoints without affecting payment processing

Changes

• Added ~40 lines of clear, practical documentation
• Included working code examples for both approaches
• Emphasized separation of concerns: endpoint serving vs. payment settlement
• Reduced developer confusion about endpoint selection

User Impact

This change helps developers by:

• ✅ Clearly explaining the endpoint discovery mechanism
• ✅ Providing examples for both production and development scenarios
• ✅ Reducing confusion about where payments are settled
• ✅ Preventing misconfiguration errors
• ✅ Making endpoint migration between environments easier
• ✅ Improving developer experience and trust

Related

• Issue docs: Inconsistent API endpoint URLs across x402 docs #229: Inconsistent API endpoint URLs across x402 docs
• PR docs: clarify x402 endpoint discovery in SDK docs #236: Complementary endpoint clarification approach
• Docs: https://docs.opengradient.ai/developers/x402/

Fixes #229