fix(cargo): include resolver config in .cargo/config.toml

[iunanua]

What does this PR do?

Change the cargo resolver behavior

It controls what happens when a dependency's latest version declares a rust-version higher than your workspace's MSRV.

Two modes:

• "allow" (default): Cargo always picks the newest semver-compatible version, even if it requires a newer compiler than your MSRV. Build then fails with a confusing compiler error.
• "fallback" (what's set here): Cargo prefers the newest version whose declared rust-version is ≤ your workspace MSRV. It falls back to an older release of that crate rather than picking one you can't compile.

Motivation

libdd-trace-utils depends on httpmock@^0.8.0-alpha.1, and the latest httpmock@0.8.3 version raised MSRV to 1.88.
cargo update pulls 0.8.3 version breaking the build (of the tests) because the tracer's MSRV is 1.87.0

[datadog-official]

 

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 1 Pipeline job failed

pr-name | pr_name_lint     

🛟 This job is unlikely to succeed on retry. Please review your pipeline configuration.

PR title linting failed: subject must not be sentence-case, start-case, pascal-case, upper-case

ℹ️ Info

No other issues found (see more)

🧪 All tests passed
❄️ No new flaky tests detected

Useful? React with 👍 / 👎

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: d22e7eb | Docs | Datadog PR Page | Give us feedback!}