Convert Rust testing workflow to reusable workflow

[MintsInc]

Context

This PR is the first step in a larger initiative to standardize CI workflows across all Datadog API client repositories. By converting our Rust testing workflow to a reusable workflow, we can:

• Enable centralized CI management: The datadog-api-spec repo will be able to use
  the same testing workflow
• Prepare for MergeQueue: Centralizing the CI in the datadog-api-spec repo is necessary to enable the MergeQueue

This PR serves as a proof-of-concept to validate that the reusable workflow approach works correctly before rolling it out to other client repositories and the specification repo.

This implementation follows the same pattern as the Go client changes in DataDog/datadog-api-client-go#3317.

Changes

New Files

• .github/workflows/reusable-rust-test.yml: A reusable workflow that contains the same logic as the original test workflow

Modified Files

• .github/workflows/test.yml: Simplified to call the reusable workflow

Key Design Decisions

• Behavior preservation: Every aspect of the original workflow is maintained through input parameters
• Configurability: The reusable workflow accepts inputs for all major parameters (Rust versions, platforms, test scripts)
• Environment variables: Git author information is inherited from the caller workflow

Tests

This PR maintains identical CI behavior for this repository - same triggers, job matrix, conditional logic, and status reporting. After merging, we'll verify that the workflow is properly triggered on other PRs, then we'll test calling this reusable workflow from the datadog-api-spec repository to verify cross-repository execution and MergeQueue compatibility before rolling out to other client repositories.