Add __str__() methods to response types for human-readable protocol messages

[bokelley]

Summary

Add __str__() methods to AdCP response types to generate consistent, human-readable messages for protocol layers (MCP, A2A, REST).

Motivation

When building AdCP implementations, response types need human-readable messages for:

• MCP tool results (displayed to users)
• A2A task messages (agent communication)
• REST API responses (optional message field)

Currently, each implementation must define these messages independently, leading to inconsistent messaging across the ecosystem.

Proposed Changes

Add __str__() methods to these response types:

Response Type	Message Pattern
GetProductsResponse	"Found N products that match your requirements."
ListCreativeFormatsResponse	"Found N creative formats."
ListAuthorizedPropertiesResponse	"Found N authorized publisher domains."
CreateMediaBuyResponse	"Media buy {media_buy_id} created successfully."
UpdateMediaBuyResponse	"Media buy {media_buy_id} updated successfully."
SyncCreativesResponse	"Creative sync completed: N created, N updated, N deleted."
GetMediaBuyDeliveryResponse	"Delivery data for N media buys."
GetSignalsResponse	"Found N signals matching your criteria."
ActivateSignalResponse	Status-based: "Signal activated successfully" / "Signal activation in progress" / etc.
ListCreativesResponse	"Found N creatives." or "Found N creatives, showing M."

Example Implementation

class GetProductsResponse(AdCPBaseModel):
    products: list[Product]
    # ... other fields

    def __str__(self) -> str:
        count = len(self.products)
        if count == 0:
            return "No products matched your requirements."
        elif count == 1:
            return "Found 1 product that matches your requirements."
        else:
            return f"Found {count} products that match your requirements."

Benefits

1. Consistency - All AdCP implementations return the same messages
2. No schema changes - __str__() doesn't add fields to the JSON schema
3. Protocol-agnostic - Works for MCP, A2A, and REST
4. Agent-friendly - Clear, conversational messages for AI agents

Reference Implementation

These patterns are currently implemented in the adcp-sales-agent in src/core/schema_adapters.py.