fix(dotnet): forward CustomAgentsLocalOnly in session.create and session.resume

[syf2211]

Summary

Forward CustomAgentsLocalOnly from SessionConfig / ResumeSessionConfig into the .NET SDK's session.create and session.resume wire payloads.

Motivation

Fixes #1888. Agent discovery runs during session initialization. The .NET SDK previously only sent customAgentsLocalOnly via the post-create session.options.update call, which arrives too late to restrict discovery to the working directory. The Go SDK already includes this field in session.create / session.resume.

Changes

• Add customAgentsLocalOnly to internal CreateSessionRequest and ResumeSessionRequest records
• Populate the field from config.CustomAgentsLocalOnly in CreateSessionAsync and ResumeSessionAsync
• Add unit serialization tests (serialize when set, omit when null)
• Add E2E wire-capture tests for create and resume requests

Tests

• dotnet test test/GitHub.Copilot.SDK.Test.csproj --filter FullyQualifiedName~GitHub.Copilot.Test.Unit — 155 passed
• New serialization tests for customAgentsLocalOnly — passed
• E2E wire-capture tests added (same harness as existing ClientOptionsE2ETests; require tsx in CI)

Notes

• Post-create session.options.update behavior is unchanged; this only ensures the value is present at initialization time.
• Pre-submit review (composer-2.5): APPROVE 4/5 — minimal fix matching Go SDK behavior for explicitly configured values.

Fixes #1888