feat: replace product-context prefix hack with SCRT2 session-context API

[chintandesai-cimulate]

Summary

Replace the text-prefix hack (Viewing product details for: <id>) with the proper SCRT2 v2 session-context API. Product context is now sent as a single commerce_agent_context variable (Object / StructuredValue) alongside the message, leaving the shopper's message text untouched.

What changed

• AgentforceClient.sendMessage(text, contextVariables?) — accepts optional context variables and wraps them in a SessionContextSet envelope on the /conversation/{id}/message endpoint.
• buildPdpInlineContext(productId) replaces withProductContext() — builds a single commerce_agent_context variable as a StructuredValue containing:

  {
    "version": "1.0.0",
    "page_context_type": "pdp_inline",
    "page_context_data": { "id": "<productId>" }
  }

• The agent declares commerce_agent_context as an Object type variable, parses page_context_type, and transitions to the appropriate topic accordingly.
• Product ID is resolved fresh on every send (agent clears external context after each turn) via the same resolveProductId() strategies (query param, path pattern, PWA fallback, SFRA DOM).
• SCRT2 warning responses (message accepted but context variables dropped) are logged.
• Default capabilitiesVersion changed to "65" — the minimum that supports session-context variables.
• New SDK types exported: SessionContextTextValue, SessionContextStructuredValue, SessionContextVariable, CommerceAgentContext.
• demo/index.html updated with product-id-param="pid", capabilities-version="65", and a demo script to seed the pid query param.

Why StructuredValue instead of TextValue

The SCRT2 API silently drops Object-typed variables sent as TextValue (soft rejection — no error, the variable just disappears). Using StructuredValue ensures the BYON agent receives the context as a native object.

Test plan

• npm run typecheck — clean
• npm test — 195 passed (13 test files)
• npm run build — ESM + UMD both succeed
• End-to-end: sent a message on a PDP page, verified the StructuredValue context envelope in the network request, agent parsed page_context_type and transitioned to product_inline_pdp topic
• Manual: send a message on a non-PDP page and verify no context field in the request body