Accept typed chat continuations

[chubes4]

Summary

• accept canonical typed tool call/result continuations without requiring synthetic user text
• map paired A2A data parts into canonical message envelopes while rejecting duplicate, orphaned, or mismatched pairs
• constrain typed input to paired tool messages so callers cannot inject arbitrary policy or assistant roles

Fixes #514.

Verification

• composer smoke
• composer phpstan
• composer validate --strict
• git diff --check

AI assistance

OpenCode with openai/gpt-5.6-sol was used to trace the WPCOM continuation contract, implement the generic mapping and validation, and run deterministic verification. Chris Huber directed the work and is responsible for every line.