fix(core): Serialize errorCode for plain HttpException responses

[hktitof]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

a plain HttpException built with a string body and an errorCode option sets exception.errorCode, but BaseExceptionFilter rebuilds the response body from getResponse() and drops the code, so these two behave differently today:

// built-in exceptions (fixed by #17615): errorCode IS in the body
throw new BadRequestException('Password is too weak', { errorCode: 'WEAK_PASSWORD' });
// -> { message, error, statusCode, errorCode }

// plain HttpException: errorCode is LOST
throw new HttpException('Forbidden', 403, { errorCode: 'FORBIDDEN_ERR' });
// -> { statusCode: 403, message: 'Forbidden' }  <- errorCode missing

this is the sibling gap the author of #17615 called out in that PR: "A plain new HttpException('Forbidden', 403, { errorCode }) is unchanged, since its response is a bare string and BaseExceptionFilter rebuilds the body itself... I can add it here if you want it"

Issue Number: N/A (no dedicated issue, this completes the follow-up offered in #17615)

What is the new behavior?

BaseExceptionFilter keeps errorCode in the rebuilt body when the exception response is a bare string:

{
  "statusCode": 403,
  "message": "Forbidden",
  "errorCode": "FORBIDDEN_ERR"
}

nothing changes when no errorCode is set, and object responses are still passed through verbatim

Does this PR introduce a breaking change?

• Yes
• No

Other information

verified: 2 new cases in exceptions-handler.spec.ts, the errorCode one fails without this change, the no-errorCode one asserts the body shape stays untouched, full exceptions suites pass (26 files / 209 tests), prettier + oxlint clean