feat(csharp): implement get_snapshot method in C# SDK

[atharvalade]

Which issue does this PR close?

Closes #2627

Rationale

The C# SDK is missing the get_snapshot API method, which is needed for feature parity with the Rust client.

What changed?

The C# SDK had no way to capture system state snapshots. Both TCP and HTTP clients now lacked the GetSnapshotAsync method, and the SnapshotCompression/SystemSnapshotType enums used implicit 0-based values that didn't match the Rust binary wire protocol codes.

Added GetSnapshotAsync to the IIggySystem interface with TCP (binary serialization) and HTTP (JSON + binary ZIP response) implementations. Assigned explicit integer values to both enums to match Rust's as_code()/from_code() mappings. Added an integration test validating ZIP archive responses for both protocols.

Local Execution

Passed, all 55 unit tests pass, SDK and integration test projects build with 0 warnings/0 errors
Pre-commit hooks not ran, prek not installed, this is a C#-only change (no Rust modifications)

AI Usage

Claude Opus 4.6
Minimal, used for general function scaffolding and verifying binary format alignment against the Rust source
Verified by cross-referencing every enum value, byte layout, and JSON format against the Rust server source code byte-by-byte; ran full build and unit test suite locally
Yes I can explain every line

[spetz]

Thank you for the contribution, I think it looks ok, but @lukaszzborek please confirm too :)

[atharvalade]

The C# test failure is a transient NuGet restore issue (NU1301: 301 Moved Permanently for System.Configuration.ConfigurationManager). Not related to this PR — all C# lint, build, e2e (231 tests), BDD, and examples checks passed. Could a maintainer re-run the failed job?

[lukaszzborek]

Yes, it's looks good. In CI was some restore problem, please rerun it