test: add source-audit tests for the async DoclingLoader rework

[jannefleischer]

THIS IS MOSTLY DONE BY AI. I am an urban planner having (almost) no understanding of the inner workings of tests...

This relies on PR open-webui/open-webui#26947 (which is part of discussion in open-webui/open-webui#26931)

Covers the DoclingLoader rework that replaces the old blocking /v1/convert/file call with submit (/v1/convert/file/async) -> long-poll (/v1/status/poll/{task_id}) -> retrieve (/v1/result/{task_id}), on both its sync (load()) and true async (aload()) paths, plus:

• optional overall timeout (DOCLING_SERVE_TIMEOUT), wired end-to-end from config.py through LOADER_CONFIG_KEYS/RETRIEVAL_CONFIG_KEYS, ConfigForm, and update_rag_config
• optional status_callback hook invoked on submit and on task_position updates while polling, on both paths
• load_from_task_id() / aload_from_task_id() to resume an already-submitted task without re-uploading
• _notify_status_async(): the async path awaits status_callback's return value if it's awaitable, since a real caller may reasonably supply an async def callback (e.g. one that awaits a DB write) — calling it as a plain function would silently produce an un-awaited coroutine that never runs
• Loader.aload()'s dispatch now routes DoclingLoader through its true async aload() directly instead of asyncio.to_thread(self.load, ...), so a slow conversion no longer occupies a worker thread from the shared default executor for its entire duration; every other loader is unaffected and still goes through asyncio.to_thread

Source-level tests (regex-match the actual backend source via the open_webui_backend fixture) rather than importing open_webui.retrieval.loaders.main directly, since that module pulls in a dependency tree (typer, langchain_community, azure.identity, ftfy) not present in this repo's test environment — same approach already used for the knowledge-status tests.

One thing worth calling out for review: method-boundary lookaheads in this file use \n (?:async\s+)?def\s+\w+ rather than \n def\s+\w+. DoclingLoader interleaves sync and async methods, and a lookahead recognizing only plain def runs straight past an async def boundary, sweeping a later method's body into the match — I caught this via a false-positive failure on the load_from_task_id test itself once the async methods were added after it in the source.

Verified: all 37 tests pass against the feat-docling-loader-async source branch, and all 37 correctly fail against the pristine, pre-rework dev source — confirmed by pointing OPEN_WEBUI_SOURCE_DIR at each in turn, so these are discriminating regression guards rather than tautologies, not just "assert True".

ruff format / ruff check (line length 100, per this repo's pyproject.toml) are clean.