docs: correct toAi() reference and document conversational context

[lmajano]

Summary

• Rewrites the-basics/routing/routing-dsl/ai-routing.md's toAi() section, which predated the real implementation and described a fictional API surface:
  • toAi( target, [name] ) → corrected to the real single-argument toAi( runnable ) signature
  • coldbox.system.web.routing.IAiRunnable with invoke()/stream()/batch()/info() actions taking (event, rc, prc) → corrected to the real bx-ai IAiRunnable (run()/stream()/getName()/getDescription())
  • Fictional {"prompt": ..., "context": ...} / {"response": ..., "model": ..., "tokens": ...} bodies → corrected to the real {"input": ..., "params": {}, "options": {}} / {"output": ..., "success": true} shape
• Adds a new Conversational Context section documenting the userId/conversationId/threadId resolution that toAi()'s invoke/stream/batch sub-routes now perform, merging resolved context into options and echoing threadId back via the JSON body, an X-Thread-Id header, and a leading SSE thread frame.
• The toMCP() section and the closing "Using Both Together" section were verified accurate against the current implementation and are unchanged.

Source of truth for the conversational-context behavior: ColdBox/coldbox-platform#681

Why

Two issues, addressed together to avoid leaving corrected content next to inaccurate content on the same page:

1. The pre-existing toAi() documentation had drifted from the actual Router.cfc implementation well before this change.
2. coldbox-platform PR #681 added conversational-context resolution to toAi()'s sub-routes, which needed documenting.

Validation

• Confirmed both internal links resolve relative to the page's location: ../../event-handlers/server-sent-events.md → the-basics/event-handlers/server-sent-events.md, and ../../../digging-deeper/ai/agentic-coldbox.md → digging-deeper/ai/agentic-coldbox.md. Both files exist in the repo.
• Confirmed the #conversational-context anchor link targets the new ### Conversational Context heading added later on the same page.
• Searched the repo for other pages linking to anchors within ai-routing.md (e.g. ai-routing.md#invoke-endpoint) — none found.
• No SUMMARY.md changes needed — content-only edit to an already-listed page.

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