…own and defined-but-unmapped status codes (dotnet#126602)

`SmtpException.GetMessageForStatus` fell through `default` to
`SmtpStatusCode.CommandUnrecognized`, so any unrecognized status value
(e.g. `(SmtpStatusCode)0` from an uninitialized `_statusCode`) produced
the misleading message *"Syntax error, command unrecognized."* —
implying an SMTP 500 response when none occurred.

## Description

- **`Strings.resx`**: Added three new resource strings:
- `SmtpUnknownStatusCode` → `"Unknown SMTP status code."` (for truly
undefined status code values)
- `SmtpCannotVerifyUserWillAttemptDelivery` → `"Cannot verify user, but
will attempt delivery."` (for SMTP 252)
- `SmtpGeneralFailure` → `"General failure."` (for the internal
`GeneralFailure = -1` sentinel)
- **`SmtpException.cs`**: Separated `default` from `case
SmtpStatusCode.CommandUnrecognized` in `GetMessageForStatus`, and added
explicit case labels for all previously unmapped defined enum members
(`CannotVerifyUserWillAttemptDelivery` and `GeneralFailure`):
  ```csharp
  // Before
  default:
  case SmtpStatusCode.CommandUnrecognized:
      return SR.SmtpCommandUnrecognized;

  // After
  default:
      return SR.SmtpUnknownStatusCode;
  case SmtpStatusCode.GeneralFailure:
      return SR.SmtpGeneralFailure;
  case SmtpStatusCode.CannotVerifyUserWillAttemptDelivery:
      return SR.SmtpCannotVerifyUserWillAttemptDelivery;
  case SmtpStatusCode.CommandUnrecognized:
      return SR.SmtpCommandUnrecognized;
  // ... all other defined enum members explicitly handled
  ```
- **`SmtpExceptionTest.cs`**: Added assertion confirming unknown status
codes yield a message distinct from `CommandUnrecognized`.

## Customer Impact

Exceptions thrown with undefined `SmtpStatusCode` values (e.g. during
connection failures before any SMTP response is received) produce a
confusing "command unrecognized" message, making diagnosis harder —
particularly when the real cause is a transport/handshake failure
unrelated to SMTP 500. All defined `SmtpStatusCode` enum members now map
to their own descriptive messages; the `default` case exclusively
handles truly undefined values.

## Regression

No — this behavior has been present since the original .NET Framework
implementation.

## Testing

Existing `SmtpExceptionTest` suite (318 tests, 0 failures). Added
assertion to `TestConstructorWithStatusCodeArgument` verifying that `new
SmtpException((SmtpStatusCode)666).Message` differs from `new
SmtpException(SmtpStatusCode.CommandUnrecognized).Message`.

## Risk

Very low. Logic change in a `switch` statement; only affects diagnostic
messages. All previously handled enum values retain their existing
messages. The two previously unmapped defined enum values
(`CannotVerifyUserWillAttemptDelivery`, `GeneralFailure`) now return
descriptive messages instead of the misleading "command unrecognized"
text.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rzikm <32671551+rzikm@users.noreply.github.com>