Add RAII lease handle (LeaseScopeAsync) + leak metric; move Smtp.Pool tests out of CI

[marklauter]

Summary

Two independent, additive changes on top of the already-merged metrics modernization:

1. RAII lease handle — an ergonomic, leak-resistant way to consume the pool.
2. Smtp.Pool test relocation — moves the sample's test projects under samples/ and out of the CI gate, plus some CI housekeeping.

No breaking changes in this PR. (The metrics rename to OTEL conventions already landed on main as metrics modernization; this branch builds on it.)

1. RAII lease handle (LeaseScopeAsync)

using var lease = await pool.LeaseScopeAsync(cancellationToken);
await lease.Item.SendAsync(message, cancellationToken);

• LeaseScopeAsync (extension over the existing public LeaseAsync/Release) returns a disposable Lease<TPoolItem>; the using returns the item on scope exit.
• Idempotent, single-shot return (interlocked) — immune to the double-release footgun.
• If the pool was disposed while the item was out on lease, the lease disposes the orphaned item itself (closes the "dispose leased items yourself" footgun).
• Leak observability: a Lease garbage-collected without being disposed is recorded on a new pool.leases.leaked counter (published on the MSL.Pool meter, tagged pool.name). The finalizer is suppressed up front in Dispose, so an unexpected Release failure still propagates but is never miscounted as a leak.
• Purely additive — no change to Pool, IPool, or IPoolMetrics; works with any IPool implementation. The Smtp.Pool sample's send path now uses it.

2. Move Smtp.Pool tests into samples/, out of the CI gate

• Smtp.Pool.Tests and Smtp.Pool.Integration.Tests move from tests/ to samples/ alongside the sample (git mv, history preserved).
• Removed from Pool.slnx, so neither CI workflow runs them (both test Pool.slnx; the integration suite needs Docker and shouldn't gate the library).
• New samples/Smtp.Pool.slnx (sample + its tests + Pool) keeps them runnable locally as a unit.
• Includes CI housekeeping staged alongside: a setup-dotnet composite action, a dependabot config, and workflow trigger/cleanup updates.

Verification

• dotnet test Pool.slnx → 94 tests, only Pool.Tests (smtp suite no longer in the CI solution).
• dotnet build samples/Smtp.Pool.slnx → full graph, 0 warnings.
• Relocated samples/Smtp.Pool.Tests → 43 pass, coverage above its floor (root Directory.Build.props still applies).
• Full solution builds 0 warnings under AnalysisMode=all + warnings-as-errors; Pool.Tests coverage 100% line / 92% branch / 100% method (above the 95/90/95 ratchet).

Not run here: the smtp4dev integration suite — it requires Docker (Testcontainers). Compile-verified only; intentionally no longer in CI.

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