Expose ResultCode and DiagnosticMessage on SearchResult

[Pujathacker2210]

Problem

GetLDAPError returns nil when resultCode is 0 (success), discarding
the diagnosticMessage from SearchResultDone. Per RFC 4511 §4.1.9, servers
may set diagnosticMessage even on a successful operation to communicate
additional information such as degraded state.

For example, Red Hat IPA sets diagnosticMessage when the directory is
reinitializing but still returns resultCode 0 with an empty entry list.
Callers of Search() currently have no way to see that message — they only
see zero entries and no error, which is indistinguishable from a genuine
empty result.

Solution

• Add ResultCode and DiagnosticMessage fields to SearchResult.
• In Search(), populate them from the SearchResultDone packet (case 5) before calling GetLDAPError, so they are preserved even whenGetLDAPError returns nil on success.
• Add ParseLDAPResult helper that extracts resultCode, matchedDN, and diagnosticMessage from an LDAPResult BER packet without the early return on success that GetLDAPError has.
• Copy the new fields in appendTo so paged searches preserve them.

Backward compatibility

• GetLDAPError is unchanged.
• SearchResult is a struct; adding fields is backward compatible in Go.
• Existing callers are unaffected — they never read the new fields.
• Callers who need the diagnostic on success can now read result.DiagnosticMessage after Search() returns.

References

• RFC 4511 §4.1.9 — LDAPResult definition (resultCode + diagnosticMessage)
• RFC 4511 §4.5.2 — SearchResultDone is an LDAPResult

Prior art

Python's python-ldap library already exposes the diagnostic message after successful operations by calling get_option(OPT_DIAGNOSTIC_MESSAGE) on the LDAP handle (source). This PR brings the same capability to go-ldap by surfacing diagnosticMessage on the SearchResult struct regardless of resultCode.

[Pujathacker2210]

@t2y @cpuschma @johnweldon - Request you to review the PR