Address review follow-ups on the async enclave attestation gate - #4621

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The enclave provider async tests derive from EnclaveProviderBase and call
ThreadRetryCache.Remove, where ThreadRetryCache is a MemoryCache. That type
lives in Microsoft.Extensions.Caching.Memory, so the test assembly needs a
direct compile reference to it.

In Project mode the reference flows transitively through the SqlClient project
reference, which is why local builds passed. In Package mode the SqlClient
package reference sets ExcludeAssets="compile" so that the compiler binds
against the implementation assembly rather than the ref assembly, and that
exclusion also suppresses the transitive compile asset. The result was:

  SqlColumnEncryptionEnclaveProviderAsyncShould.cs(570,21): error CS0012:
  The type 'MemoryCache' is defined in an assembly that is not referenced.

Declaring the PackageReference explicitly fixes Package mode and is accurate
in both modes, since the test code really does compile against that type. The
version resolves through central package management, which already selects
8.0.1 or 9.0.18 based on the target framework.

Verified by reproducing the failure locally in Package mode without this
change and confirming it builds and passes with it, on net8.0, net9.0 and
net10.0.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: cf57fdc0-1c77-40f1-a1d5-7c0bd98c1a89

Addresses review feedback on the async enclave provider hierarchy:

- The async attestation gate is now taken and released entirely inside
  CreateEnclaveSessionAsync via a disposable lease, so the semaphore is
  released in a finally on every exit path (success, failure, cancellation)
  and ownership never depends on which thread a continuation resumes on.
- The async path no longer reads or writes the static, thread-id keyed
  ThreadRetryCache, so it can no longer leave stale entries that would make
  a later synchronous caller skip the sync gate.
- Concurrent cold starts still collapse into a single attestation because
  CreateEnclaveSessionAsync re-checks the session cache after taking the gate.
- GetEnclaveSessionHelperAsync now early-returns a cached session and performs
  no gating, so it never waits on another caller's in-flight attestation.
- The signing key retry backoff is awaited (Task.Delay) on the async path
  instead of blocking a thread pool thread with Thread.Sleep; the synchronous
  path keeps its existing Thread.Sleep behaviour.
- Documented the net462 cancellation granularity limit in MakeRequestAsync.
- Tests: renamed the mismatched cancellation test, added coverage for gate
  release on attestation failure, and tightened the concurrent cold-start
  assertion to exactly one attestation.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Follow-up to review feedback on the async enclave provider hierarchy.

- EnclaveProviderBase now seals CreateEnclaveSessionAsync. It acquires the
  async gate, re-checks the session cache once held, and releases in a
  finally; providers supply only protocol-specific logic via the new
  protected abstract CreateEnclaveSessionCoreAsync. The gate can no longer
  be bypassed or leaked by a derived provider, which also removes the
  publicly-reachable lease type and its disposal contract.

- EnclaveProviderBase also seals GetEnclaveSessionAsync, routing through
  GetEnclaveSessionHelperAsync via the new GeneratesNonceForAttestation
  hook. This fixes a latent bug: NoneAttestationEnclaveProvider had no
  async overrides, so it inherited the default that calls the *synchronous*
  GetEnclaveSession, taking the sync gate that only a later synchronous
  CreateEnclaveSession would release. An async caller never makes that
  call, so the sync gate was stranded for its full 15s timeout. Covered by
  a new regression test that fails (15s stall) without the seal.

- NoneAttestationEnclaveProvider: extracted the session-setup parsing into
  a shared helper used by both the sync and async paths.

- Removed the GetAttestationParametersAsync/InvalidateEnclaveSessionAsync
  overrides from the Azure and VSM providers; they were byte-for-byte
  identical to the inherited defaults.

- Documented that the async path collapses the attestation service call but
  deliberately does not collapse the per-caller work before it, in the
  source comment, the doc snippet, and a new assertion on
  AttestationParametersCount.

- Tests: drive the real three-call sequence (including
  GetAttestationParametersAsync, which produces the client ECDH key) in the
  Attest/AttestAsync helpers; reuse one provider instance in the
  gate-release-on-failure test so it does not depend on the gate being
  static; assert the mixed sync/async race converges on a single cached
  session.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

- Drop the now-unused Microsoft.Data.Common usings from the Azure and VSM
  providers, left behind when their redundant overrides were removed.
- Use the TimeSpan overload of Task.Delay in both async retry loops.
- Correct the ThreadRetryCache comment: it records thread IDs for the sync
  path, not the attestation url and nonce.
- Document that the async gate deliberately does not share the sync path's
  adaptive lock timeout, so neither path can degrade the other.
- Make the async gate timeout overridable so tests can drive the timeout
  fallthrough without waiting out the production timeout.
- Add tests for the gate timeout fallthrough and for async re-attestation
  after a session is invalidated, plus a concurrency high-water mark on the
  fake so collapsing and fallthrough are asserted directly rather than by
  timing.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 148862f6-f66a-4a89-8436-ec4a008bbea3