feat: add TestContext.Isolation interface for test resource isolation

[thomhurst]

Summary

• Adds a new ITestIsolation interface on TestContext with UniqueId, GetIsolatedName(), and GetIsolatedPrefix() helpers
• Extracts isolation functionality from WebApplicationTest so any test can create unique resource names (database tables, queue topics, cache keys) without requiring a specific base class
• WebApplicationTest now shares the same atomic counter, ensuring unique IDs across all test types

Changes

Action	File
Create	TUnit.Core/Interfaces/ITestIsolation.cs
Create	TUnit.Core/TestContext.Isolation.cs
Modify	TUnit.Core/TestContext.cs
Modify	TUnit.AspNetCore/WebApplicationTest.cs
Create	TUnit.UnitTests/TestIsolationTests.cs
Modify	docs/docs/test-lifecycle/test-context.md
Modify	docs/docs/examples/aspnet.md

Usage

// Available in any test - no base class required
var tableName = TestContext.Current!.Isolation.GetIsolatedName("todos");  // "Test_42_todos"
var prefix = TestContext.Current!.Isolation.GetIsolatedPrefix(".");       // "test.42."

Test plan

• dotnet build succeeds (TUnit.Core + TUnit.AspNetCore)
• 10/10 new unit tests pass (TestIsolationTests)
• CI pipeline passes
• Verify WebApplicationTest existing tests still pass

🤖 Generated with Claude Code

[claude]

Missing Snapshot Test Updates

This PR introduces public API changes that require updating snapshot tests per CLAUDE.md Rule 2:

Snapshot Testing - Changes to source generator output or public APIs require running snapshot tests. Commit .verified.txt files. NEVER commit .received.txt.

Public API Changes Made

1. New public interface: ITestIsolation (this file)
2. Modified TestContext: Now implements ITestIsolation and exposes public ITestIsolation Isolation => this;

Required Action

Run the public API snapshot tests and commit the updated .verified.txt files:

dotnet test TUnit.PublicAPI
# Review the changes in *.verified.txt files
git add TUnit.PublicAPI/**/*.verified.txt

See mandatory-rules.md for details on when snapshot tests are required.

[claude]

Code Review

I found an architectural issue with the dual-counter implementation that causes inconsistent behavior for users.

Issue: Inconsistent UniqueId Values

Problem: For tests inheriting from WebApplicationTest, the UniqueId property returns different values depending on whether accessed via this.UniqueId or TestContext.Current!.Isolation.UniqueId.

Root Cause: Both constructors independently increment the shared counter at different lifecycle stages:

• TestContext constructor runs during discovery/build phase (TestContext.cs:33)
• WebApplicationTest constructor runs during execution phase (WebApplicationTest.cs:21)

Impact: This causes GetIsolatedName() and GetIsolatedPrefix() to produce different resource names for the same test depending on the access path:

// In a test inheriting from WebApplicationTest:
this.GetIsolatedName("users");                              // Returns "Test_5_users"
TestContext.Current!.Isolation.GetIsolatedName("users");   // Returns "Test_1_users"

This defeats the purpose of test isolation — a user mixing both APIs in the same test will get non-matching resource names.

---

Suggested Fix: Reuse TestContext's ID

Instead of generating a separate ID, have WebApplicationTest delegate to the existing TestContext.Isolation.UniqueId:

internal WebApplicationTest()
{
    UniqueId = TestContext.Current!.Isolation.UniqueId;
}

Why this is better:

• Consistency: Both access paths return the same ID for the same test
• Single source of truth: Only TestContext generates IDs
• Simpler architecture: No dual increment logic to reason about
• Better abstraction: WebApplicationTest becomes a thin wrapper around TestContext.Isolation

This ensures that whether a user calls this.GetIsolatedName("foo") or TestContext.Current!.Isolation.GetIsolatedName("foo"), they get the same resource name.

---

Additional Consideration: Make Helper Methods Wrappers

For maximum clarity and maintainability, consider having WebApplicationTest's helper methods delegate to TestContext.Isolation as well:

protected string GetIsolatedName(string baseName) 
    => TestContext.Current!.Isolation.GetIsolatedName(baseName);

protected string GetIsolatedPrefix(string separator = "_") 
    => TestContext.Current!.Isolation.GetIsolatedPrefix(separator);

This makes WebApplicationTest a pure convenience wrapper rather than a parallel implementation, reducing duplication and preventing future drift.

---

CLAUDE.md Compliance: ✅ No violations found. Checked snapshot tests, AOT compatibility, performance, and dual-mode requirements.