Multiple @-references in one message (esp. @url: refs, each a full
web_extract round-trip) were expanded in a serial `for ref in refs: await`
loop. Switch to asyncio.gather over the independent _expand_reference calls,
reassembling warnings/blocks in original positional order so output is
byte-identical to the serial path; the token-budget check is unchanged.
Generic + provider-agnostic: helps every web backend equally (exa/tavily/
firecrawl/parallel) since it's above the provider layer. RED/GREEN test:
3 url refs @ 0.2s each = 0.60s serial -> ~0.20s concurrent.
