Add SN/I certificate support over mTLS Proof-of-Possession (PoP)

[Robbie-Microsoft]

Summary

Adds support for using a Subject-Name/Issuer (SN/I) certificate as the first-leg credential over mTLS Proof-of-Possession (PoP).

Today an SN/I cert is used to sign a private_key_jwt (x5c) client assertion, which yields a Bearer token (SNI + Bearer — unchanged). This PR lets a confidential-client app present that same certificate as the client TLS certificate in the mutual-TLS handshake to the token endpoint, so Entra ID (ESTS) returns a token that is cryptographically bound to the cert (token_type=mtls_pop, cnf/x5t#S256). The credential is identical; only the mechanism changes (assertion-signer → TLS client cert).

Also covers the 2-leg Federated Identity Credential (FIC) over mTLS PoP, where both legs are cert-bound and the final token is bound to the Leg-1 certificate thumbprint.

Public API

// Direct SN/I cert -> mTLS-bound PoP token
IAuthenticationResult result = cca.acquireToken(
        ClientCredentialParameters.builder(Set.of("https://vault.azure.net/.default"))
                .mtlsProofOfPossession()          // opt in
                .build())
        .get();

result.metadata().tokenType();            // TokenType.MTLS_POP
result.metadata().bindingCertificate();   // public material only: x5c chain + x5t#S256

// FIC Leg 2: assertion-authenticated app that must still bind to a cert
ConfidentialClientApplication.builder(clientId, ClientCredentialFactory.createFromClientAssertion(t1))
        .mtlsBindingCertificate(sniCert)
        .build();

• ClientCredentialParameters.Builder.mtlsProofOfPossession()
• ConfidentialClientApplication.Builder.mtlsBindingCertificate(IClientCertificate)
• New public types: TokenType (BEARER / MTLS_POP), BindingCertificate (public material only), plus AuthenticationResultMetadata.tokenType() / bindingCertificate().

Internal changes

• New: TokenType, BindingCertificate, MtlsClientCertificateHelper (mTLS socket factory, thumbprint, binding-cert resolution), MtlsEndpointHelper (login.* → mtlsauth.* host rewrite; region optional → global mtlsauth.microsoft.com; tenanted-authority validation; sovereign-cloud fail-fast).
• Request building (TokenRequestExecutor): direct cert path omits client_assertion; FIC Leg 2 sends client_assertion_type=...:jwt-pop; result populates tokenType/bindingCertificate.
• Transport: mTLS socket factory threaded through OAuthHttpRequest / HttpHelper / IHttpHelper via an injected mTLS DefaultHttpClient (MSAL owns the mTLS transport).
• Cache isolation: access-token cache keyed on {token_type + cert KeyId} so Bearer and PoP (and different certs) never alias.
• Telemetry: isolated TokenType.telemetryValue() (BEARER=2, MTLS_POP=6) for cross-SDK parity; the locked v5 wire header is intentionally left unchanged.

Backward compatibility

The existing SNI + Bearer (assertion) and broker SHR-PoP flows are unchanged and remain byte-for-byte identical — validated by the full existing unit suite.

Testing

• Unit: 3 new classes (MtlsEndpointHelperTest, MtlsClientCertificateHelperTest, MtlsProofOfPossessionTest) + a hermetic PKCS12 resource. Full suite: 410 tests, 0 failures, 0 errors, 0 skipped.
• E2E: MtlsPopIT — direct SNI→PoP (global + regional), Bearer/PoP cache isolation, and 2-leg FIC (both legs bound). CI-only: requires the lab SN/I cert (non-CNG) + an ESTS allow-listed resource; ESTS gates mTLS PoP on the final resource audience.

Docs / samples

• New sample confidential-client/ClientCredentialMtlsProofOfPossession.java
• changelog.txt and .github/copilot-instructions.md updated
• Javadoc on all new public API

Out of scope / follow-ups

• Managed-identity first-leg mTLS variant (separate follow-up).
• User-scoped (user_fic) FIC over mTLS PoP.
• US Gov / China clouds (fail-fast today).

Risks / open questions

• mTLS E2E only passes in CI (lab creds + allow-listed resource).
• The telemetry wire header intentionally does not yet carry token type (would require an ESTS-coordinated schema bump).