Second follow-up for salvaged PR #94547, folding in review findings from the
duplicate-PR cluster (#47015, #55054, #58476, #72977, #73685 all fix the
same 401) and the sweeper review of #73685:
- Replace the dot-anchored suffix predicate with exact-match against the
existing _ALLOWED_TEAMS_SERVICE_HOSTS allowlist (two of the five
duplicate PRs converged on this independently). Any Azure customer can
register <name>.trafficmanager.net profiles, so suffix matching was not
safe. Also requires https on the default port — :444 on an allowlisted
host no longer receives the bearer (sweeper finding on #73685).
- Stream _fetch_attachment_bytes through _read_httpx_body_with_limit
instead of buffering response.content — the shared inbound media cap now
applies to authenticated downloads too (sweeper finding: a lying
Content-Length must not OOM the gateway).
- Serialize token refresh with a lazily-bound asyncio.Lock so concurrent
attachments share one STS POST (review finding on #94547).
- Token expiry now uses time.monotonic() (from #55054) — wall-clock jumps
can't extend a stale token.
- Tests updated: exact-allowlist predicate (lookalike/subdomain/port/scheme
negatives), streaming fake client, and a concurrent-cold-cache lock test.
Mutation-checked: suffix match, silent drop, no-lock, and unbounded
buffer each fail a test.
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