chore: add "refreshable" to the docs dictionary

The Documentation Tests job fails on every PR because GoogleAuth.mdx uses
"refreshable-credential code path" and cspell doesn't know the word. Pre-existing
on main, unrelated to this branch's fix, but it gates the checks — and the
dictionary already carries the same kind of coinage (parallelizable, pasteable,
sandboxed), so the word belongs there rather than the prose being reworded.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: odgrim

docs/cspell.json

@@ -34,6 +34,7 @@
     "prek",
     "prereq",
     "pyenv",
+    "refreshable",
     "resourcemanager",
     "runbook",
     "runbooks",