feat: new SDK methods

[scmmishra]

client.contacts

Method	HTTP
contacts.contactable_inboxes(account_id, contact_id)	GET /contacts/{id}/contactable_inboxes
contacts.merge(account_id, base_contact_id, mergee_contact_id)	POST /actions/contact_merge

client.inboxes

Method	HTTP
inboxes.get_agent_bot(account_id, inbox_id)	GET /inboxes/{id}/agent_bot
inboxes.set_agent_bot(account_id, inbox_id, agent_bot_id)	POST /inboxes/{id}/set_agent_bot

client.inboxes.agents (new nested resource)

Method	HTTP
inboxes.agents.list(account_id, inbox_id)	GET /inbox_members/{inbox_id}
inboxes.agents.add(account_id, inbox_id, agent_ids)	POST /inbox_members
inboxes.agents.update(account_id, inbox_id, agent_ids)	PATCH /inbox_members
inboxes.agents.remove(account_id, inbox_id, agent_ids)	DELETE /inbox_members

All 8 methods have both sync and async (AsyncChatwootClient) equivalents.