Optimize CI caching and metrics flush

[YanagiKH]

Context

This change reduces repeated CI build time and removes an avoidable allocation in a hot telemetry flush path.

• The package test workflow now uses Rust caching so unchanged dependencies and build artifacts can be reused across runs.
• The SQLite metrics exporter now uses std::mem::take when flushing the buffered samples, which avoids draining into a newly allocated vector.

How this change came about

I reviewed the package test workflow and the SQLite telemetry exporter, then made two focused changes:

1. Added Rust cache to .github/workflows/package_test.yml to reduce repeated build work in CI.
2. Replaced drain(..).collect() with std::mem::take in src/engine/telemetry/exporter.rs to avoid an extra allocation while flushing buffered telemetry.

I made the edits directly in the repository and kept the scope limited to the two files above.

How has this been tested?

I verified the change by code inspection. No additional runtime test was run in this session.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature or improvement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-only change

Checklist:

• I have read [CONTRIBUTING.md](https://github.com/pathwaycom/pathway/blob/main/CONTRIBUTING.md),
• My code follows the code style of this project,
• If this is a substantial change, it has prior maintainer approval and the "How this change came about" section is filled in,
• I described the modification in the CHANGELOG.md file,
• My change requires a change to the documentation, and I have updated it accordingly.

[CLAassistant]

All committers have signed the CLA.