User is able to define a subagent with provider out of his own policy list

[cch-gh]

What if SUBAGENT_PROVIDER is configured in provider list but not allowed in related policy, still can be used for sub-agent?

Originally posted by @jasonli0226 in #15

[cch-gh]

User is able to define a subagent with provider out of his own policy list, but not able to invoke the subagent constrainted by the policy.