MCP tools permanently removed from session after transient server disconnect, even after reconnection (Microsoft 365 connector)

This reproduces the behavior described in #38043 and #33779, both closed as duplicates of
each other and eventually #21271 — which the original reporter closed as a false alarm
unrelated to this. Filing fresh since there's no open issue tracking it and both closed
threads point back to me doing that if still relevant.

Same MCP server (Microsoft 365/Outlook, Mail.ReadWrite + Mail.Send scopes granted), same
app version (2.1.270) throughout, no update in between.

- outlook_create_draft succeeded twice on 2026-09-07 and once more on 2026-09-13 (~9:43pm
local), the last one inside a scheduled/background routine.
- Every session checked since, including a brand-new interactive session started the next
day on the identical 2.1.270 build, has that tool missing entirely from the deferred-tool
registry — not erroring, not schema-load-failing, just absent from ToolSearch results for
its exact name.
- A fresh session does not self-heal it. Other tools on the same server (search/read) kept
working the whole time; only the write-capable ones (create/update/delete draft)
disappeared.

Happy to share more transcript detail if useful for reproduction.