test: cover EntityFrameworkOutboxStore parallel publishing throttle and per-message outcome recording

[Vulthil]

Summary

• Adds direct coverage of the real EntityFrameworkOutboxStore parallel-publishing throttle (EnableParallelPublishing / MaxDegreeOfParallelism), which previously had no tests against the real store — the existing coverage only exercised fakes that mirrored the semaphore pattern.
• ThrottlesParallelDispatchToTheConfiguredMaxDegreeOfParallelism: a gate-based ConcurrencyTrackingDispatcher parks dispatches until the cap is saturated, proving peak concurrent dispatches reach and never exceed the configured MaxDegreeOfParallelism — deterministically, with no timing sleeps.
• RecordsEachMessageOutcomeIndividuallyWhenAParallelBatchHasAFailure: one failing dispatch in a parallel batch leaves the other rows' outcomes intact (ProcessedOnUtc set, RetryCount 0, no error) while the failing row records RetryCount 1 and its error without being dead-lettered.
• Extends the NewStore helper with optional parallel-publishing parameters; test-only change, no src/ modifications.

Verification

• Full solution build with 0 errors (warnings-as-errors on).
• Built test executable run with --filter-class on the store test class for net9.0 and net10.0: 7/7 passed on both.
• Full Vulthil.SharedKernel.Outbox.EntityFrameworkCore.Tests project: 12/12 on both TFMs, plus 3 repeat runs of the class to confirm the concurrency test is stable.

Backport to v1.0: no

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.