[management] Check a vendor change, and bound the check by one deadline

Two gaps a review surfaced.

Moving a record from one catalog provider to another changed neither field the
check looked at, so an unchanged credential started being offered to a
different vendor, under a different auth header, with nothing asking whether it
was accepted there.

And each host lookup built its own eight-second budget from a background
context, so a slow resolver could spend one before the request spent another,
and a caller that gave up was still waiting. Bedrock made that three: it now
checks its runtime host as well. One deadline is taken at the top of Fetch and
carried through both lookups and the request.

The create description also had the exemption backwards, reading as though an
upstream the check cannot reach is stored unverified. Unreachable blocks; only
what cannot be checked at all is exempt.

shared/management/http/api/openapi.yml

@@ -14146,7 +14146,7 @@ paths:
       description: |
         Connects a new Agent Network AI provider for the account.
 
-        The upstream URL and credential are checked against the vendor before the provider is stored, so a record that cannot reach its vendor is refused rather than saved. Returns 422 with a message naming whichever of the two is at fault. A catalog provider with no listing endpoint, and one whose upstream the check cannot reach from the management service, are stored without being checked.
+        The upstream URL and credential are checked against the vendor before the provider is stored, so a record that cannot reach its vendor is refused rather than saved. Returns 422 with a message naming whichever of the two is at fault — a rejected credential, an upstream that does not resolve or answer, a vendor outage, and a timeout all block the write. Only what cannot be checked at all is exempt and stored unverified: a catalog provider with no listing endpoint, one with no host to derive a listing from, and an upstream resolving to a private address the management service will not dial.
       tags: [ Agent Network ]
       security:
         - BearerAuth: [ ]
@@ -14213,7 +14213,7 @@ paths:
       description: |
         Update an existing Agent Network AI provider.
 
-        When the upstream URL or the API key changes, the pair is checked against the vendor before the change is stored, and a refusal returns 422 without replacing what was there. An update omitting the API key is checked against the stored one. Edits touching neither field are stored without a check.
+        When the upstream URL, the API key or the catalog provider changes, the record is checked against the vendor before the change is stored, and a refusal returns 422 without replacing what was there. An update omitting the API key is checked against the stored one. Edits touching none of the three — a rename, model rows, price edits — are stored without a check, as are the cases the create description lists as unverifiable.
       tags: [ Agent Network ]
       security:
         - BearerAuth: [ ]