Close remaining gateway multi-connection test gaps from #114

[takashikasuya]

Summary

Follow-up to #114 (closed via #115). Re-auditing the four test files that issue touched turned up two remaining depth/coverage gaps within the same scope:

• GatewayIngressServiceTest: ゲートウェイgRPC多重接続・ポイントリスト配信のゲートウェイ間分離テスト追加 #114's own gap analysis said Ingress had zero multi-gateway concurrency coverage; the fix landed with a two-gateway test (StreamTelemetry_TwoGatewaysConcurrently_NoCrossContamination), but GatewayEgress got a three-gateway depth test (Command_RoutesCorrectly_WithThreeConcurrentGateways). Adds StreamTelemetry_ThreeGatewaysConcurrently_NoCrossContamination to bring Ingress to the same depth as Egress.
• GatewayProvisioningControllerTest: the full-fetch path has a two-gateway isolation test (Returns200_TwoRegisteredGateways_EachSeesOnlyOwnPoints), but the diff (?since=<etag>) path — which reads from a single shared IGatewayPointListSnapshotStore instance across all gateways in production — had no equivalent isolation test. Adds Diff_SnapshotIsolatedPerGateway_CrossGatewaySinceFallsBackToFull, asserting that one gateway presenting another gateway's (real, retained) etag as since correctly falls back to its own full point list instead of resolving against the wrong gateway's snapshot. (Verified by inspection that MemoryGatewayPointListSnapshotStore's cache key already includes gatewayId — this is a coverage gap, not a live bug — but the isolation now has a test that would go red if that scoping ever regressed.)

Both additions strictly follow the existing patterns in each file (same helper methods, same fake/mock setup, same assertion style) — no production code changes.

Test plan

• dotnet test --filter "FullyQualifiedName!~IntegrationTest" — could not run locally: this environment's egress policy blocks builds.dotnet.microsoft.com (used by dotnet-install.sh), so no .NET SDK is available here. Both new tests were written by strictly mirroring already-passing sibling tests in the same files (same helpers, same fake types, same assertion patterns/overloads) and reviewed carefully for compile-correctness against those exact signatures.
• The automatic .NET Build & Test job in pr-check.yml (fires on every PR, unlike the manual-trigger-only oss-ci/integration-tests) will build and run these tests — I'll watch it and fix forward if it flags anything.

Checklist

• I read CONTRIBUTING.md and followed the coding conventions in CLAUDE.md
• I added or updated tests for the change — this PR is test-only
• I updated relevant docs (README.md, docs/) if behavior or setup changed — n/a (test-only, no behavior/setup change)

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