fix(translation): rewrite colliding anthropic tool-call ids (#502)

[jsvisa]

A backend that reuses OpenAI tool-call ids across responses (e.g. Grok emitting call_0, call_1, call_0) made Switchyard translate that conversation to Anthropic with duplicate tool_use ids. Anthropic returns 400 "tool_use ids must be unique", and clients pair results to calls by id, so agent loops never terminated.

Translation now treats id uniqueness as an Anthropic-facing invariant: first-seen ids pass through unchanged; a repeat mints sydup2_call_0-style ids (distinct from the sy64_ scheme, original readable after stripping the prefix). The Anthropic request decoder records the ids a conversation already used in request extensions, and both buffered and streaming response encoders seed from that — no new cross-request state. Every tool_result is rewritten to match its own call.

Verified: new buffered/streaming/extension tests fail before the fix and pass after; full translation suite (231 tests), cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings clean.

Fixes #502