docs: add Agent API v2 endpoints

[mintlify-development]

Summary

• Adds documentation for new Agent API v2 endpoints with fire-and-forget job creation
• Includes create job, get job status, and send message endpoints
• New v2 API returns JSON immediately for polling-based status retrieval
• Adds localized pages for French, Spanish, and Chinese

Changes

• Created admin-openapi-v2.json with v2 API specification
• Added MDX documentation pages in api/agent-v2/
• Updated navigation in docs.json for all languages
• Updated API introduction to mention v2 endpoints

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Note

Low Risk
Documentation-only changes (OpenAPI + MDX + nav updates) with no runtime logic or security-sensitive code paths modified.

Overview
Adds a new admin-openapi-v2.json spec for Agent API v2, defining async job creation (POST /agent/{projectId}/job), status polling (GET /agent/{projectId}/job/{id}), and follow-up messaging (POST /agent/{projectId}/job/{id}/message).

Publishes new API reference pages for these v2 endpoints (EN plus fr/, es/, zh/) and updates api/introduction.mdx and docs.json navigation to surface the new Agent v2 section alongside the existing streaming v1 docs.

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[ethanpalm]

closing for #4225