Summary

• Extends GonkaProvider with chat_with_tools and chat_typed<T> over the Gonka ECDSA-signed transport
• Promotes decode_tool_chat_response to pub(crate) on OpenAiProvider with a provider_name parameter
• Adds pub(crate) build_typed_chat_body<T> helper extracted from OpenAiProvider::chat_typed
• Error bodies truncated before logging (256 chars) and InvalidInput messages (512 chars) to prevent unbounded memory from malicious nodes
• 2 new wiremock tests; all 35 gonka tests pass; test(gonka_tools) filter matches 2 tests per acceptance criterion

Test plan

• cargo nextest run -p zeph-llm --features gonka -E 'test(gonka_tools)' — 2 tests pass
• cargo nextest run -p zeph-llm --features gonka -E 'test(gonka)' — 35 tests pass
• cargo clippy --workspace --features full -- -D warnings — clean
• cargo +nightly fmt --check — clean
• cargo nextest run --workspace --lib --bins — 8892 tests pass
• cargo doc --no-deps --features gonka -p zeph-llm — no broken intra-doc links
• Live API round-trip (LLM serialization gate) — requires Gonka testnet credentials; to be verified by CI cycle before merge

Notes

• signed_request swallows 429 inside the retry loop (converts to ApiError { status: 429 }), so RateLimited path is intentionally not present in these methods
• Usage tracking syncs from inner tracker to self.usage after decode_tool_chat_response via last_usage()/last_cache_usage()

Closes #3612