feat: add handshake Exception on ITlsHandshakeFeature

[DeagleGross]

Adding handshake exception on the ITlsHandshakeFeature

In order to allow users to get the exception happened during tls handshake, adding new approved API on ITlsHandshakeFeature and populating it in the HttpsConnectionMiddleware.

Since we now assume ITlsHandshakeFeature can be used after tls handshake failed, and underlying sslStream may be disposed (should be actually), I added a way to snapshot all the fields from sslStream to TlsConnectionFeature backup-fields, which will be used if set.

Note: changed to NET11_0_OR_GREATER instead of NETCOREAPP to not make an unintended change in net10

Fixes #65758

[Copilot]

Pull request overview

This PR adds a new Exception property to ITlsHandshakeFeature so middleware can inspect TLS handshake failures, and updates Kestrel’s HttpsConnectionMiddleware to populate it (including snapshotting SslStream-backed properties before disposal).

Changes:

• Add ITlsHandshakeFeature.Exception (default interface member) as a new approved API surface.
• Populate the handshake exception in HttpsConnectionMiddleware on handshake failures and snapshot TLS-related properties before SslStream disposal.
• Add functional tests validating Exception is set on handshake failure/timeout and remains null on success; update sample to demonstrate usage.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/Servers/Kestrel/test/InMemory.FunctionalTests/HttpsConnectionMiddlewareTests.cs	Adds coverage for handshake exception + post-failure feature access patterns (with some flakiness risks).
src/Servers/Kestrel/samples/SampleApp/Startup.cs	Demonstrates reading ITlsHandshakeFeature.Exception after handshake completion.
src/Servers/Kestrel/Core/src/Middleware/HttpsConnectionMiddleware.cs	Sets feature.Exception on handshake failure and snapshots properties before disposal.
src/Servers/Kestrel/Core/src/Internal/TlsConnectionFeature.cs	Adds snapshotting/cached backing fields and an Exception property implementation.
src/Servers/Connections.Abstractions/src/PublicAPI/net11.0/PublicAPI.Unshipped.txt	Declares the new public API entry for net11.0.
src/Servers/Connections.Abstractions/src/Features/ITlsHandshakeFeature.cs	Introduces the new Exception property (NETCOREAPP) with a default implementation.

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