[Fusion] Expose the operation plan on PolicyEvaluationContext

[michaelstaib]

Ticket: hc-fork-2-jzk.10 (follow-up from your #24 review)

PolicyEvaluationContext takes the OperationPlan in place of string OperationId. It is exposed as Plan, and callers use Plan.Operation.Id. The constructor null-checks the plan. No other member changes.

The tests build a real plan through the FusionTestBase compose and plan helpers, with no stubs.

Verified:

• dotnet build src/All.slnx: 0 errors.
• Authorization tests: 108/108 on net8 through net11.
• After I rebased onto the merge of Add the authenticated, requiresScopes and policy source schema directives #23, I re-ran the net10 Authorization tests: 27/27.

Minor review notes, not fixed:

• PolicyTestHelper composes and plans on every CreateContext call without caching. The suite still runs in about 2s.
• PlanFactory subclasses FusionTestBase only to reach its protected static helpers.

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