Keep Rust PRs green with a minimal CI gate

Add a focused GitHub Actions workflow for pull requests into main plus
manual dispatch. The workflow checks workspace formatting and runs the
rusty-claude-cli crate tests so we get a real signal on the active Rust
surface without widening scope into a full matrix.

Because the workspace was not rustfmt-clean, include the formatting-only
updates needed for the new fmt gate to pass immediately.

Constraint: Keep scope to a fast, low-noise Rust PR gate
Constraint: CI should validate formatting and rusty-claude-cli without expanding to full workspace coverage
Rejected: Full workspace test or clippy matrix | too broad for the one-hour shipping window
Rejected: Add fmt CI without reformatting the workspace | the new gate would fail on arrival
Confidence: high
Scope-risk: narrow
Directive: Keep this workflow focused unless release requirements justify broader coverage
Tested: cargo fmt --all -- --check
Tested: cargo test -p rusty-claude-cli
Tested: YAML parse of .github/workflows/rust-ci.yml via python3 + PyYAML
Not-tested: End-to-end execution on GitHub-hosted runners

rust/crates/api/src/providers/anthropic.rs

@@ -322,7 +322,10 @@ impl AnthropicClient {
                     .with_property(
                         "estimated_cost_usd",
                         Value::String(format_usd(
-                            response.usage.estimated_cost_usd(&response.model).total_cost_usd(),
+                            response
+                                .usage
+                                .estimated_cost_usd(&response.model)
+                                .total_cost_usd(),
                         )),
                     ),
             );