Add four spec-required environment variable fields to the Settings class
that were previously missing, causing AttributeError when accessed:
- server_url: str | None — maps to CLEVERAGENTS_SERVER_URL (spec: server.url)
- server_token: str | None — maps to CLEVERAGENTS_SERVER_TOKEN (spec: server.token)
- format: str | None — maps to CLEVERAGENTS_FORMAT (spec: core.format)
- default_estimation_actor: str | None — maps to CLEVERAGENTS_DEFAULT_ESTIMATION_ACTOR
(spec: core.default_estimation_actor)
All fields default to None when the corresponding env var is unset, and
are fully statically typed (no type: ignore). BDD scenarios cover env var
binding, default value, type validation, and model_fields presence for
each new field.
ISSUES CLOSED: #2866
