decode extended responseName by context class

[netliomax25-code]

1. In Conn.Extended the ExtendedResponse children are matched on child.Tag only: tag 10 sets Name and tag 11 sets Value.
2. The optional responseName is [10] context-class, but the resultCode that always precedes it is a universal ENUMERATED with the same tag number, so the result code matches the name case. When a server omits responseName (the usual case), ExtendedResponse.Name ends up holding the raw result code octet ("\x00" on success) instead of an empty string.
3. Skip non-context-class children before the tag switch, so only the context-tagged responseName/responseValue are read. This mirrors the class-aware decoding already used for other controls.

Verified with a regression test that drives Extended over the in-package fake connection with a success response that carries a responseValue but no responseName; it asserts Name is empty and fails on the current code (Name == "\x00").

[cpuschma]

@netliomax25-code Thank you for yor PRs. LGTM, but please take a look at the linting errors. Thank you.

[netliomax25-code]

Fixed. The two errcheck hits were the unchecked Close() returns in the new test's defers; wrapped both to discard the error explicitly. Lint and the Go 1.25/1.26 validation jobs are green now, and the regression test still passes with -race.