Add a public RSQL date-window builder (retargeted to date_creation)

[baraline]

Gap

The date_creation window expression is hand-concatenated at three sites: _statistics.py:394-395,
:521-522, :734-735. There is no tested place where the date grammar lives.

Retarget: date_creation, not date_mod

The originally proposed changed_since(dt) names a field this codebase never filters on -- date_mod
appears only as a sort value. It would deduplicate zero existing call sites.

date_mod is a top-level scalar datetime on GetTicket, so such a filter would very likely work; it is a
legitimate forward-looking capability (incremental sync) with no current in-repo caller. But the helper
that pays for itself today is a date_creation window (ge + end-of-day le), which dedupes all
three sites with no live instance required.

Note rsql_all_filter could already have joined those two clauses -- the duplication is partly
self-inflicted.

Placement

The existing six RSQL helpers live in _async/clients/commons/_filters.py and are all private --
glpi_python_client.__all__ (83 names) contains no filter, escape or rsql name. That module is under
_async/, so it is duplicated into the generated tree; adding a public pure-string API there would make it
un-exportable without picking a tree.

Precedent for a tree-neutral home: GlpiEnum.rsql_equals (models/api_schema/enums.py:33) is tree-neutral,
exported from the root, and its docstring states the intent -- "the helper methods keep RSQL string
generation on the public surface." A new glpi_python_client/rsql.py follows that.

Two side-fixes worth doing here

• Document the GLPI v2 silent-drop behaviour in _filters.py's module docstring and in the skills. It is
  currently recorded only in _statistics.py:47, :154-155, :739, test_statistics.py:282 and
  integration_tests/test_integration.py:478-480 -- nowhere a filter author would look.
• Switch the hand-built filters at integration_tests/test_integration.py:205,257 to the helpers.

Not a bug

The unquoted embedded space in the current expressions is accepted by live GLPI 11 (the integration tests
drive it through get_ticket_statistics(default_days=...)). Worth tidying, but do not bill it as a fix.

A module-level function is not gated by test_skill_references.py, which scans dir(GlpiClient).