ObservabilityPipeline cardinality processor schema breaking changes + new budget forecast endpoint

BreakingBreaking change

This is a breaking change — integrations that rely on the affected behaviour will need updates before the change rolls out.

What changed

Several breaking changes to the ObservabilityPipelineTagCardinalityLimitProcessor schemas, plus a new additive endpoint:

Breaking: The mode field in ObservabilityPipelineTagCardinalityLimitProcessorPerMetricLimit is renamed to override_type and is now required; the old ObservabilityPipelineTagCardinalityLimitProcessorPerMetricMode schema (with values tracked/excluded) is removed and replaced by ObservabilityPipelineTagCardinalityLimitProcessorOverrideType (values limit_override/excluded).
Breaking: The mode field in ObservabilityPipelineTagCardinalityLimitProcessorPerTagLimit is renamed to override_type (now required); the old ObservabilityPipelineTagCardinalityLimitProcessorPerTagMode schema is removed.
Breaking: A new required field tracking_mode is added to the top-level ObservabilityPipelineTagCardinalityLimitProcessor object — existing payloads omitting it will fail validation.
Additive: A new GET /api/v2/cost/budgets/{BudgetID}/custom_forecast endpoint (GetCustomForecast) is introduced to retrieve a budget's custom forecast.