feat: add coderd_ai_group_budget resource

[ssncferreira]

Description

Adds the coderd_ai_group_budget resource, which manages the AI spend limit that AI Gateway applies to each member of a group (/api/v2/groups/{group}/ai/budget). coderd_ai_user_budget_override follows in #454.

Changes

• Add coderd_ai_group_budget with group_id and spend_limit, registered under the "AI Governance" docs subcategory.
• spend_limit is in integer micro-units, like coderd_ai_model_price, and is validated against codersdk.MaxAISpendLimitMicros at plan time.
• Import by group ID or <organization-name>/<group-name>, like coderd_group.
• Add an example, import docs, mock-server plan tests, and an acceptance test.

Decisions

• Standalone resource instead of an attribute on coderd_group: budgets often target groups that Terraform does not manage (IdP-synced groups, Everyone), and the budget API requires the AI Gateway entitlement while groups do not.
• Follows coderd_ai_model_price for unmanaged values: the plan fails when the group already has a different budget, and an equal budget is adopted.
• No computed id, following feat!: remove redundant id from coderd_agents_default_model and coderd_ai_model_price #452. group_id forces replacement.
• Destroy deletes the budget. A 404 on read removes the resource from state, since Coder deletes the budget with its group.
• A 404 on upsert means the group does not exist or the deployment is older than v2.34.0, so the error names both.

Related to AIGOV-682

Note

Generated by Coder Agents on behalf of @ssncferreira.

[linear-code]

AIGOV-682