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

[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 ID to be resumed with a new session object (which obstructed fixing the former).

[github-actions]

Cross-SDK Consistency Review ✅

This PR fixes a .NET-specific bug in ResumeSessionAsync: the old code used _sessions.TryAdd() which threw InvalidOperationException when resuming a session ID already tracked by the client. The fix replaces the session via AddOrUpdate and restores the previous session on RPC failure.

This change brings .NET into alignment with all other SDKs:

SDK	Session registration in resumeSession	Behavior on duplicate
Node.js	this.sessions.set(sessionId, session)	Silent overwrite ✅
Python	self._sessions[session_id] = session	Silent overwrite ✅
Go	c.sessions[sessionID] = session	Silent overwrite ✅
Java	sessions.put(sessionId, session)	Silent overwrite ✅
Rust	self.sessions.lock().insert(...)	Silent overwrite ✅
.NET (before)	_sessions.TryAdd()	❌ Threw InvalidOperationException
.NET (after)	_sessions.AddOrUpdate()	Silent overwrite ✅

No cross-SDK consistency issues found. The PR correctly aligns .NET behavior with all other implementations.

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Generated by SDK Consistency Review Agent for #1986 · 64.1 AIC · ⌖ 7.56 AIC · ⊞ 5K · ◷