Accept *Async suffix on saga method names (#2578)

[jeremydmiller]

Fixes #2578.

The bug

HandlerDiscovery already treats saga methods named StartAsync / HandleAsync / OrchestrateAsync / ConsumeAsync / StartOrHandleAsync / NotFoundAsync as valid — it strips the Async suffix when matching against the canonical name list (HandlerDiscovery.cs:54-55). But SagaChain.findByNames matched on strict string equality, so async-suffixed methods were:

• discovered into the handler graph (so the saga thinks it handles the message),
• silently dropped from StartingCalls / ExistingCalls / NotFoundCalls,
• compiled into a generated chain that constructs the saga but never invokes the user's method,
• persisted with Saga.Id == Guid.Empty,
• failed on insert with ArgumentOutOfRangeException: You must define the saga id when using the lightweight saga storage.

The reporter's observation: the failure mode is uniquely confusing because the documentation already mentions StartAsync as if it works (docs/guide/durability/sagas.md:374).

The fix

SagaChain.findByNames now matches both the bare name and its async-suffixed twin:

return Handlers
    .Where(x => x.HandlerType.CanBeCastTo<Saga>()
                && methodNames.Any(n =>
                    x.Method.Name == n || x.Method.Name == n + "Async"))
    .ToArray();

This is option 1 from the issue's proposed fixes — making SagaChain symmetric with HandlerDiscovery's convention.

Tests

Bug_2578_saga_async_method_names covers both halves:

Discovery-level — StartingCalls / ExistingCalls / NotFoundCalls are populated for *Async methods across every saga convention (StartAsync, HandleAsync, OrchestrateAsync, ConsumeAsync, StartOrHandleAsync, NotFoundAsync).

End-to-end — StartAsync and HandleAsync are actually invoked by the generated handler, saga state mutates correctly, and persists in InMemorySagaPersistor.

8 new tests. All 8 fail on main, all 8 pass with this change. All 1352 other CoreTests still pass (1360 total).

Docs

docs/guide/durability/sagas.md "Method Conventions" section now documents the *Async variants alongside the existing convention table, with a small code sample.

Test plan

• New Bug_2578_* tests fail without the fix
• New Bug_2578_* tests pass with the fix
• All other saga-related CoreTests pass (87 total)
• Full CoreTests pass (1360 total)

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