fix: cancel stale completion requests

[atirna]

Description

I cancel outstanding completion requests when a document-changing notification arrives. Completion declaration loops now check for cancellation, so stale requests release before the document refreshes.

Fixes #3251

Why

Large generated C bindings can make completion enumeration expensive. The response should reflect the latest document instead of waiting for earlier edits to finish.

Verification

• before, on current master: a 30,000-declaration import with eight rapid textDocument/didChange and completion pairs left the newest response pending for 19.523 seconds
• after: the same session returned the newest response in 0.694 seconds
• zig build test -Dtest-filter=completion
• zig build test