rfc: signal activation DSP discovery and execution path

[bokelley]

Summary

The Signals spec has gaps in the signal activation → media buy flow. A Buyer Agent cannot reliably activate signals and execute a media buy through a Sales Agent because:

1. No SA execution path for activation keys (Signal Agent: activation_key execution mechanism undefined for Sales Agents #1944) — SA receives an activation_key but the spec doesn't define how to map it to DSP-level targeting
2. DSP mismatch risk (DSP mismatch: activate_signal destination may not match SA's DSP #1945) — BA may activate signals on one DSP while the SA routes through a different one
3. No DSP discovery (BA cannot discover SA's DSP integrations before signal activation #1946) — BA has no way to discover which DSPs a SA is integrated with before calling activate_signal

Proposal

1. Add DSP integration metadata to get_products

{
  "products": [...],
  "integrations": {
    "dsps": ["the-trade-desk", "dv360"],
    "preferred_activation": "the-trade-desk"
  }
}

2. Define activation_key execution interface

Specify how a Sales Agent maps an activation_key to DSP-level segment targeting — whether the key is opaque or carries structured metadata, and what the SA's responsibilities are.

3. Handle cross-DSP coordination

Either require BA to match activation destination to SA's DSP, or define a coordination mechanism when they differ.

Supersedes

• Signal Agent: activation_key execution mechanism undefined for Sales Agents #1944 — activation_key execution mechanism undefined
• DSP mismatch: activate_signal destination may not match SA's DSP #1945 — DSP mismatch between activation and delivery
• BA cannot discover SA's DSP integrations before signal activation #1946 — BA cannot discover SA's DSP integrations

Context

All three issues surfaced by joohwa kim (NHN ACE) during A1 certification — deep protocol analysis tracing the signal-to-media-buy flow end to end.