agents: add external-provider OAuth connection methods

[jkosanam]

Adds actor-scoped OAuth connection management to the hand-written hosted-agents client, completing the pydo side of MARSOHS-2070 (godo and doctl already carry the same surface).

What this adds

Four methods on both the sync (SessionsOperations) and async (aio) clients, surfaced as client.agents.sessions.*:

Method	Call
list_connections(provider, *, user_id=, status=, page=, per_page=)	GET /v2/agents/auth/{provider}/connections
create_connection(provider, *, user_id, scopes=None)	POST .../connections
get_connection(provider, connection_id)	GET .../connections/{id}
delete_connection(provider, connection_id)	DELETE .../connections/{id}

These sit in the hand-written custom layer next to the existing start_provider_auth / poll_provider_auth, matching the actor-scoped connection shape harness-api proxies (field names mirror tool-registry's public connection: user_id is the actor, status is pending/active/expired).

Scope / compatibility

Additive only (432 insertions, 0 deletions). The legacy team-wide start_provider_auth / poll_provider_auth flow is untouched.

Testing

• 14 new unit tests (7 sync + 7 async) covering filter forwarding, create body, get/delete paths, and required-argument validation.
• Full sync + async session suites pass (94 tests).
• black clean; source within the 88-char pylint limit.

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