Authenticate to the Johnson Controls Metasys REST API v6 and keep the JWT alive with refreshToken

Documented steps

1. Confirm the target is a Metasys Site Director running Metasys 14.0 or later. API v6 is the only version supported on releases 14, 14.1, 15 and 16; v5 is deprecated on 14/14.1 and unavailable from 15 onward. Check https://jci-metasys.github.io/api-landing/guides/version-support-matrix/ before coding.
2. Build the base URL as https://{hostname}/api/v6 where {hostname} is the Site Director host. The OpenAPI servers block declares exactly this: url 'https://{hostname}/api/v6'. The doc labels v6-16 / v6-15 / v6-14-1 are documentation revisions of API v6 only - the wire path segment is always 'v6', never 'v6-16'.
3. Create or obtain a Metasys account whose Access Type is set to API. The login schema states only accounts with an Access Type of API can request a token from the REST API; a normal operator account returns 403.
4. POST {base}/login with Content-Type: application/json and body {"username":"...","password":"..."}. For an Active Directory account use the form name@domain.
5. Read the 200 response (media type application/vnd.metasysapi.v6+json): {"accessToken":"eyJ...","expires":"2021-04-22T00:12:18Z"}. Store both fields; 'expires' is an ISO-8601 UTC instant, not a duration.
6. Send every subsequent request with header Authorization: Bearer {accessToken}. The only exception is the SSE stream endpoint, which also accepts ?access_token={accessToken} as a query parameter.
7. Schedule a refresh well before 'expires': GET {base}/refreshToken with the current Bearer token. It returns the same {accessToken, expires} shape. Refreshing requires a still-valid token - there is no refresh-token grant to fall back on once the token expires.
8. On expiry or 401, fall back to a fresh POST /login and re-establish any open streams and subscriptions, which are bound to token lifetime.
9. Reference: https://jci-metasys.github.io/api-landing/api/v6-16 (Metasys REST API, 'Version 6 for 16.0', OpenAPI 3.0.3). Doc set index: https://jci-metasys.github.io/api-landing/ | changelog: https://jci-metasys.github.io/api-landing/guides/api-changelog/ | version support: https://jci-metasys.github.io/api-landing/guides/version-support-matrix/ | polling and streaming limits: https://jci-metasys.github.io/api-landing/guides/attribute-access-guidelines/