WWW-Authenticate parsing matches a field name as a substring of another auth-param

Describe the bug

Ran into this wiring up an OAuth-protected MCP client. When a 401 WWW-Authenticate challenge carries several auth-params, extract_field_from_www_auth (mcp/client/auth/utils.py) matches the requested field name as a substring of another param's name, so a different param shadows the real value.

The pattern is rf'{field_name}=(?:"([^"]+)"|([^\s,]+))' searched with re.search, with no boundary before field_name.

To reproduce

import httpx
from mcp.client.auth.utils import extract_field_from_www_auth, extract_resource_metadata_from_www_auth

def r(h):
    return httpx.Response(401, headers={"WWW-Authenticate": h},
                          request=httpx.Request("GET", "https://api.example.com/"))

# A real `scope` is present, but `error_scope` shadows it:
print(extract_field_from_www_auth(r('Bearer error_scope="decoy", scope="read write"'), "scope"))
# -> 'decoy'   (expected 'read write')

# Only a decoy param exists; no real `scope`:
print(extract_field_from_www_auth(r('Bearer custom_scope="leaked"'), "scope"))
# -> 'leaked'  (expected None)

# Same for resource_metadata, which drives discovery URL selection:
print(extract_resource_metadata_from_www_auth(r('Bearer x_resource_metadata="https://decoy.example.com"')))
# -> 'https://decoy.example.com'  (expected None)

Expected behavior

The field name should match only as a complete auth-param name (at the header start or after a whitespace/comma separator), so scope doesn't match inside error_scope. This matters most for resource_metadata, since the client uses it to pick the protected-resource-metadata discovery URL — reading it from the wrong param points discovery at the wrong place.

Environment

• mcp main (reproduces on v1.x as well)

I have a small fix (anchor the param name to the header start or a separator) plus regression cases added to the existing test_extract_field_from_www_auth_* parametrized tests, and can open a PR if this looks right.

Reviewed by AI

從 mcp/client/auth/utils.py 中的 extract_field_from_www_auth 開始，檢查現有的參數化測試 test_extract_field_from_www_auth_*。重現包含 error_scope 和 custom_scope 等誘餌參數名稱的案例，然後驗證解析器只會回傳完整的 auth-param 名稱，且 resource_metadata 探索會忽略誘餌參數。