Add MSI mTLS PoP support: pure MI + FIC-with-MI (impl for devex #3832)

[gladjohn]

Summary

Implements the devex proposal in #3832: mTLS Proof-of-Possession for Managed Identity (pure MSI) and FIC backed by Managed Identity (federated identity credentials).

Triggered by setting AuthorizationHeaderProviderOptions.ProtocolScheme = `MTLS_POP`` on the downstream API options (together with RequestAppToken = true). No new public API.

Devex (the entire dev-facing surface)

json "AzureKeyVault": { "RequestAppToken": true, "ProtocolScheme": "MTLS_POP", "AcquireTokenOptions": { "ManagedIdentity": { "UserAssignedClientId": "..." } } }

For FIC-with-MI, add the SignedAssertionFromManagedIdentity credential under AzureAd.ClientCredentials — the binding cert minted by MI flows through to the outer CCA automatically.

What changed (~165 LOC of real code)

File	Change
TokenAcquisition.cs	Chain WithMtlsProofOfPossession().WithAttestationSupport() on the pure-MI builder when IsTokenBinding is set
ConfidentialClientApplicationBuilderExtension.cs	Dispatch FIC-with-MI to a bound-assertion delegate returning ClientSignedAssertion (carrying the binding cert). All other signed-assertion source types still throw IDW10115
ManagedIdentityClientAssertion.cs	New internal GetSignedAssertionWithBindingAsync — stateless, no shared mutable flag
Microsoft.Identity.Web.Certificateless.csproj	Reference Microsoft.Identity.Client.KeyAttestation 4.84.1-preview
Properties/InternalsVisibleTo.cs	IVT for Microsoft.Identity.Web.TokenAcquisition (3rd entry, consistent with existing precedent)

Why internal + IVT (not public)

The new method's return type is MSAL's ClientSignedAssertion — exposing it would tie ID Web's public surface to MSAL's API shape forever. Keeping it internal honors the devex spec's "no new public API" goal and avoids two methods on ManagedIdentityClientAssertion that consumers might confuse.

Why not the previous design (mutable IsTokenBinding property)

The prior approach (#3773) added a public IsTokenBinding { get; set; } flag on ManagedIdentityClientAssertion that callers had to set before invoking GetSignedAssertionAsync. That has three issues:

1. Mutating a DI singleton from request-scoped code = thread-safety race
2. Permanent public API for purely internal plumbing
3. The base GetSignedAssertionAsync returns string, so the binding cert was effectively dropped

This PR replaces it with a stateless delegate that returns ClientSignedAssertion (assertion + cert pair), so the binding cert flows correctly into MSAL.

Tests

• 2 new unit tests in WithClientCredentialsTests.cs:
  • WithBindingCertificateAsync_FicWithManagedIdentityAssertion_ReturnsBuilder — dispatch succeeds
  • WithBindingCertificateAsync_FicWithFileBasedAssertion_StillThrows — IDW10115 regression guard
• Full suite (net8.0): 930 passed, 11 skipped, 0 failed
• Pre-existing isTokenBinding cert-based tests unchanged and still pass

Samples

Two new daemon samples under tests/DevApps/daemon-app/:

• daemon-app-msi-mtls — pure MI mTLS PoP
• daemon-app-fic-mtls — FIC-with-MI mTLS PoP

Builds

• Microsoft.Identity.Web.TokenAcquisition (all 6 TFMs): 0 warn / 0 err
• Both samples: 0 warn / 0 err

Linked

Closes #3832 (devex)

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