feat(imp): handle COUNTER accept/decline workflows and proposal updates

[TDannhauer]

Summary

• Address PR review feedback on COUNTER accept/decline handling in IMP iTIP
• Catch Horde_Exception with logging on counter-decline errors
• Use HordeString instead of legacy Horde_String in DECLINECOUNTER mail body
• Use use import for Variables in the ItipRequest test stub

Motivation

Review on #67 requested tighter exception handling, modern string API usage, and import style in new code.

Changes

lib/Ajax/Imple/ItipRequest.php

• counter-decline: catch Horde_Exception instead of Exception; log via Horde::log($e, Horde_Log::ERR) before user notification
• _sendDeclineCounter(): HordeString::wrap() with use Horde\Util\HordeString

Expected exceptions on counter-decline:

• Horde_Exception — missing attendee address, identity lookup, MIME assembly/send failures

test/Imp/Stub/ItipRequest.php

• Add use Horde\Util\Variables for the handle() union type parameter

Test plan

• vendor/bin/phpunit -c phpunit.xml.dist test/Imp/Unit/Ajax/Imple/ItipRequestCounterTest.php
• Manual: open COUNTER mail in IMP — accept/decline actions unchanged
• Regression: normal REQUEST accept/decline/tentative unchanged

Review responses

Comment	Resolution
Do not catch random Exception without logging	Catch Horde_Exception; log with Horde::log() before notification
Prefer HordeString over Horde_String	HordeString::wrap() in _sendDeclineCounter()
Use use instead of FQCN in stub	use Horde\Util\Variables

[TDannhauer]

PR adapted.