Fix .NET in-process E2E transport coverage

[roji]

The .NET E2E fixture explicitly selected TCP even in the in-process matrix, so those tests never exercised the FFI transport and hid FFI-specific behavior.

This change:

• Allows resuming the same session in .NET SDK, aligning it with the other SDK's behavior (.NET specifically had a check against doing that). This was a blocker for some .NET E2E tests in inproc mode.
• selects RuntimeConnection.ForInProcess() for the in-process matrix while preserving TCP coverage in the other matrix
• restores the native host environment before each test because the shared fixture outlives per-test environment isolation
• aligns same-client session resume with the other SDKs by replacing the routed session wrapper before the resume RPC, without adding operation-wide locking or concurrent-resume rejection
• keeps TCP-only multi-client behavior intact and adds fixture policy and session lifetime coverage
• uses a checkpoint number valid for the native runtime's u32 domain

The complete .NET in-process suite passes through FFI with 672 tests passed and 3 existing skips. Targeted TCP resume coverage and all .NET target-framework builds also pass.

[Copilot]

Pull request overview

Updates .NET E2E coverage to exercise the FFI transport and supports same-client session resumption.

Changes:

• Selects in-process or TCP transport according to the test matrix.
• Restores host environment state between in-process tests.
• Adds same-client resume replacement behavior and related coverage.

Show a summary per file

File	Description
dotnet/src/Client.cs	Replaces tracked wrappers during resume.
dotnet/test/Harness/E2ETestFixture.cs	Selects matrix-appropriate transport.
dotnet/test/Harness/E2ETestContext.cs	Restores in-process environment state.
dotnet/test/Harness/E2ETestBase.cs	Prepares tests and chooses resume client.
dotnet/test/E2E/SessionE2ETests.cs	Covers same-client resume replacement.
dotnet/test/E2E/RpcWorkspaceCheckpointsE2ETests.cs	Uses a native-compatible checkpoint value.
dotnet/test/Unit/E2ETestFixtureTests.cs	Tests fixture transport selection.
dotnet/test/Unit/ClientSessionLifetimeTests.cs	Tests replacement and failure lifetimes.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Medium

[github-actions]

✅ Cross-SDK Consistency Review

This PR touches only the .NET SDK (dotnet/). The key behavioral change — allowing ResumeSessionAsync to replace an already-tracked session instead of throwing — is consistent with how every other SDK handles this:

SDK	Behavior when resuming a session the same client already tracks
Node.js	this.sessions.set(sessionId, session) — silently replaces (client.ts:1714)
Python	self._sessions[session_id] = session — silently replaces (client.py:3099)
Go	c.sessions[sessionID] = session — silently replaces (client.go:1265)
Java	sessions.put(sessionId, session) — silently replaces (CopilotClient.java:747)
.NET (before)	Threw InvalidOperationException — was the only outlier
.NET (after PR)	Replaces, restores previous on failure — now consistent ✅

No other SDKs need updates. The PR brings .NET into alignment with the established cross-SDK contract.

Generated by SDK Consistency Review Agent for issue #1986 · sonnet46 804.7K · ◷

[roji]

@SteveSandersonMS noticed this while working on other stuff, take a look - we weren't doing actual FFI in the .NET SDK, plus the .NET SDK was unique in not allowing the same session to be resumed with a new session object (which obstructed fixing the former).