refactor(csharp): rewrite integration tests to remove DependsOn

[atharvalade]

Remove all DependsOn attributes from 11 test files. Each test now creates its own resources with unique names via Guid.NewGuid(), ensuring full isolation and eliminating ordering dependencies.

Delete 16 unused fixture, helper, and model files that were only needed by the old DependsOn-based test structure.

Which issue does this PR close?

Closes #2654

Rationale

The DependsOn attribute creates ordering dependencies between tests, which leads to flaky failures and makes it hard to run tests in isolation. Independent tests are more reliable and easier to debug.

What changed?

Tests relied on DependsOn to run in a fixed sequence, sharing resources created by earlier tests. If one test failed, all downstream tests would skip or fail regardless of their own correctness.

Each test now creates its own stream, topic, and other resources with unique Guid-based names, then cleans up or simply leaves them isolated. The 16 fixture, helper, and model files that only existed to support the old shared-state pattern were removed as dead code.

Local Execution

• Passed
• Pre-commit hooks not ran (non-Rust changes only, prek targets Rust code)

AI Usage

1. Opus 4.6
2. Used for repetitive boilerplate and verifying no dangling references after deletions
3. Verified by building locally (0 warnings, 0 errors) and running dotnet format
4. Yes