fix(tests,#7225): rewrite MultiConnectorTests for SK 1.78 + PlanJsonHelpers xUnit suite

[jsboige]

fix(tests,#7225): rewrite MultiConnectorTests for SK 1.78 + PlanJsonHelpers xUnit suite

Context

Axe 3+ of #7225 tracks the SK 1.78 migration in the semantic-fleet submodule. The
SK 1.78 release removed Microsoft.SemanticKernel.Planning.Plan, Plan.FromJson,
Plan.State, Plan.Steps, and SequentialPlanner.CreatePlanAsync. The legacy
MultiConnectorTests exercised those APIs directly and was therefore <Compile Remove>'d
from IntegrationTests.csproj in PR #64 (c.699), breaking CI builds on main.

PR c.699 (commit 3c975a0) re-modelled samples/Plans/*.json to the new SK 1.78
ChatHistory-style format and updated the Skip reasons in MultiConnectorTests.cs,
but it did not actually rewrite the test methods themselves. This PR completes that
work.

What this PR does

1. Rewrites MultiConnectorTests.cs against the new
   Plan-as-KernelFunction pattern. Each test now:

   • Calls PlanJsonHelpers.BuildChatHistoryFromPlanJsonAsync(planPath, input) to get
     a (ChatHistory history, IReadOnlyList<ResolvedInvocation> invocations) tuple.
   • Registers a single runtime KernelFunction (via KernelFunctionFactory.CreateFromMethod)
     that walks the resolved invocations against the configured Kernel and returns a
     Task<string> (SK 1.78 auto-wraps this in FunctionResult).
   • Iterates ChatHistory to find the AuthorRole.System message (SK 1.78 removed
     the ChatHistory.SystemMessage convenience property).
   • Replaces Json.Encode(Json.Serialize(...)) with JsonSerializer.Serialize(...).

2. Drops <Compile Remove="Connectors\MultiConnector\MultiConnectorTests.cs" /> from
   IntegrationTests.csproj. The file now compiles cleanly under net8.0 + SK 1.78.

3. Adds PlanJsonHelpersTests.cs — 4 xUnit tests covering the SK 1.78 fixtures
   in samples/Plans/SK178/:

   • LoadPlan_Summarize_ResolvesExpectedShape — asserts plan-level name/description
     • the single invocation's metadata.
   • LoadPlan_SummarizeTopicsElementAt_ResolvesChainedInvocations — asserts the
     3-step chain (Summarize → Topics → ElementAtIndex) including the $RESULT__TOPICS
     argument binding.
   • BuildChatHistoryFromPlan_EmitsSystemAndUserMessages — asserts the system+user
     message split.
   • BuildInvocationPlan_ResolvesVariableSubstitutionForChainedPlan — asserts that
     step 3 receives a <RESULT__TOPICS> sentinel at plan-resolution time (the
     actual value flows through at runtime via FunctionChoiceBehavior.Auto()).

4. Adds samples/Plans/SK178/Summarize.json + Summarize_Topics_ElementAt.json —
   brought in from commit c627e40 (already implemented in that branch but not yet
   on the test-rewrite branch).

5. Adds <None Update="Connectors\MultiConnector\Plans\SK178\*.json"> entries to
   IntegrationTests.csproj so the fixtures copy to bin/.../Connectors/MultiConnector/Plans/SK178/.

Build / test verification

cd dotnet/src/IntegrationTests
dotnet build -c Debug
# → 0 errors, 4 warnings (all pre-existing, unrelated to this PR)

dotnet test -c Debug --no-build --filter "FullyQualifiedName~PlanJsonHelpersTests"
# → 4 passed, 0 failed

dotnet test -c Debug --no-build
# → 11 total: 4 passed (PlanJsonHelpersTests) + 7 skipped (Oobabooga / ChatGPT live-server tests)

The skipped tests are the original Oobabooga and ChatGPT integration tests that
require live LLM endpoints (not available in CI); they were already Skip-marked
before this PR. The build itself — which was previously broken — now compiles cleanly.

Acceptance for #7225 (Axe 3+)

• CI build no longer broken on main.
• MultiConnectorTests compiles against SK 1.78.
• PlanJsonHelpers materializes a ChatHistory + IReadOnlyList<ResolvedInvocation>
  that FunctionChoiceBehavior.Auto() can drive at runtime.
• xUnit coverage for the SK 1.78 plan format shipped.
• Step 4/4 of the multi-cycle plan: validate the cost-offload behavior end-to-end
  against a real Oobabooga server — see #7225 follow-up. Not blocking the merge
  of this PR because the build/test unblock is the highest-priority item from
  c.714.

Cross-repo plan

This PR targets the upstream MyIntelligenceAgency/semantic-fleet repo. Once
merged, a follow-up PR on jsboige/CoursIA will bump the submodule pointer. The
two-step pattern (upstream first, parent bump second) is the canonical one for
submodule-managed forks — see PR #7583 (c.706) for a prior example.

Co-author

Co-Authored-By: Claude-Code noreply@anthropic.com

Refs

• See #7225 — full Axe 3+ decision rationale (FunctionChoiceBehavior.Auto() is the
  canonical SK 1.78 path; SequentialPlanner is removed).
• Built on 3c975a0 (c.699 decision comment + Skip-reason update).
• Imports PlanJsonHelpers.cs + SK178 fixtures from c627e40.