Define provider-turn adapter contract

[chubes4]

Summary

• Adds a provider-neutral WP_Agent_Provider_Turn_Adapter contract with request/result value shapes for one model turn.
• Wires WP_Agent_Conversation_Loop to consume provider_turn_adapter while retaining loop-owned continuation, mediated tool execution, usage/request metadata aggregation, diagnostics, and typed failures.
• Adds pure-PHP smoke coverage proving fake provider-turn adapters drive the loop with and without tool mediation, without Data Machine loaded.

Closes #268.

Tests

• php tests/provider-turn-adapter-smoke.php
• php tests/conversation-loop-smoke.php
• php tests/conversation-loop-tool-execution-smoke.php
• php tests/conversation-runner-contracts-smoke.php
• composer phpstan
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the provider-turn adapter contract implementation and smoke coverage; Chris remains responsible for review and acceptance.