Add idempotent runtime tool completion

[chubes4]

Summary

• Add storage-neutral lifecycle semantics for completing runtime tool requests only while pending.
• Return retained prior runtime tool results for duplicate completions when the store exposes them, without re-running continuation.
• Refuse duplicate completions for terminal records that cannot provide a prior result, before overwriting store data.

Tests

• composer smoke
• composer phpstan
• php tests/tool-runtime-smoke.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: implemented and tested runtime tool idempotent transition semantics