Managesieve: preserve an explicitly-selected i;ascii-casemap comparator (#9981)

[MiMoHo]

Problem

When a user explicitly selects the i;ascii-casemap comparator for a filter test, the choice is silently dropped from the generated Sieve script. The saved rule therefore does not reflect the explicit selection, and re-parsing/re-saving the script loses it entirely (round-trip loss). See #9981.

Root cause

plugins/managesieve/lib/Roundcube/rcube_sieve_script.php, add_comparator() (previously ~line 1056):

// skip default comparator
if ($test['comparator'] != 'i;ascii-casemap') {
    $out .= ' :comparator ' . self::escape_string($test['comparator']);
}

Because i;ascii-casemap is the RFC 5228 default comparator, the code unconditionally suppressed it on output. However, both the parser (test_tokens()) and the engine (rcube_sieve_engine.php:1015, only sets comparator when the user picked one) populate the comparator key only on an explicit selection — an unset comparator leaves the key empty and is already short-circuited by the empty() guard at the top of add_comparator(). So the presence of comparator == 'i;ascii-casemap' unambiguously means "the user chose it", yet it was still discarded.

Fix

Emit the comparator whenever it is present. The existing empty() guard still means the default (unset) case emits nothing, so scripts without an explicit comparator are unchanged. Behavior for i;octet and i;ascii-numeric is unchanged, and i;ascii-casemap continues to be excluded from the require/extensions list (it is a built-in comparator requiring no capability).

Testing

Added two focused tests in plugins/managesieve/tests/ManagesieveScriptTest.php:

• test_explicit_ascii_casemap_comparator — parses a rule with an explicit :comparator "i;ascii-casemap", asserts as_text() emits it, and asserts it survives a re-parse round-trip. Fails on current master, passes after the fix.
• test_default_comparator_not_emitted — asserts that a rule with no explicit comparator emits no :comparator token (guards against regression).

The existing parser/parser.out fixture was updated to reflect the corrected (non-lossy) output for its explicit i;ascii-casemap case. Full ManagesieveScriptTest suite passes (35 tests). phpstan level 4 on the changed file reports no errors.

Fixes #9981