test: unblock cargo --tests build (SamplingConfig + format string drift)

[joelteply]

Summary

Three pre-existing canary breakages blocked the broader `cargo --tests` build path. These were flagged as out-of-scope in PR #1082's "Known gaps"; cleared here so the integration test build stays compilable going forward (and so regressions in any test file can land in CI signal instead of being masked behind "build broken" silence).

• `llamacpp_metal_throughput.rs` / `qwen35_live_pipeline_diff.rs`: backend `.generate(...)` signature took `temperature: f64` until the `SamplingConfig` refactor; tests still passed `0.0` / `0.7`. Updated to `SamplingConfig` literal (qwen35: explicit greedy, no repeat_penalty so output matches the bare-decode reference) and `SamplingConfig::chat()` (throughput: matches what live chat traffic uses).
• `persona_prompt_token_diagnostic.rs`: format string `"Model GGUF not present at {model_path()}"` uses Rust 2024 captured-identifier syntax which doesn't allow function calls — emits "expected `}`, found `(`" at compile time. Bound to a local + use positional `{}` with `path.display()`.

Validation class

• Contract TDD: n/a — test-only diff, no production behavior changes.
• Failure TDD: n/a — same as above.
• Performance/Resource/Residency VDD: n/a.
• Browser/UX evidence: precommit browser ping passed.
• Platform coverage: Mac M5 Pro Metal, `cargo build --features metal,accelerate --tests` clean.

Validation

• `cargo build --features metal,accelerate --tests` — clean, no errors (was 3 distinct errors before).
• `cargo test --lib --features metal,accelerate` — unchanged (pre-push gate).
• Precommit hook: TypeScript clean; browser-ping precommit test passed.

Scope discipline

This is a test-fixture-only cleanup — three files, all under `tests/`. No production source touched. Cleared from PR #1082's Known-Gaps list.