Improve WebSocket exceptions to include informative messages

[Copilot]

Closes #129192

Adds error messages for cases like receiving a text message that's not valid UTF-8, an invalid close status code / description.

The current behavior was an exception like

System.Net.WebSockets.WebSocketException (0x80004005): An exception caused the WebSocket to enter the Aborted state. Please see the InnerException, if present, for more details.

where there'd be no inner exception to inspect.

[Copilot]

Pull request overview

This PR improves diagnostic quality in System.Net.WebSockets by ensuring protocol/UTF-8 receive failures throw WebSocketException instances with informative, scenario-specific messages (instead of falling back to a generic “Aborted state” message with no inner exception).

Changes:

• Updates ManagedWebSocket receive/close error paths to pass explicit resource-based error messages into CloseWithReceiveErrorAndThrowAsync.
• Adds new SR string resources for invalid UTF-8 text payload, protocol violation, invalid close status code, and invalid close description UTF-8.
• Adds/updates tests to validate the new exception messages for invalid UTF-8 and malformed close frames.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Net.WebSockets/tests/WebSocketUtf8Tests.cs	Asserts the new invalid-UTF8 payload exception message in existing UTF-8 validation tests.
src/libraries/System.Net.WebSockets/tests/WebSocketReceiveErrorMessageTests.cs	New tests that synthesize invalid frames and validate WebSocketException.Message / InnerException.
src/libraries/System.Net.WebSockets/tests/System.Net.WebSockets.Tests.csproj	Includes the new test file in the test project compilation list.
src/libraries/System.Net.WebSockets/src/System/Net/WebSockets/ManagedWebSocket.cs	Supplies explicit error messages for invalid UTF-8 payload and malformed close payload scenarios.
src/libraries/System.Net.WebSockets/src/Resources/Strings.resx	Adds new localized resource strings used by the improved exceptions.

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