Serialize managed-prefix mixed tool responses

[mostlydev]

Summary

Serialize same-response mixed tool batches when the model emits a managed prefix followed by a runner-native suffix.

What Changed

• classify tool ownership by order instead of treating any mixed batch as invalid
• execute the managed prefix, occlude the native suffix, and ask the model to continue
• keep failing closed on native-first or interleaved mixed batches with an explicit retry message
• add unit coverage for OpenAI and Anthropic mixed-prefix serialization
• log a dedicated intervention marker for this path

Verification

• go test ./internal/proxy/...

Part of mostlydev/clawdapus#165