chore(spec): refresh spec for launch sprint + qualified enum constants

[caballeto]

Summary

Unblocks mono#369 launch sprint by handling spec changes that ripple
into TF provider source compilation.

mono#369 adds MonitorStatus enum (UP/DOWN/DEGRADED/PAUSED/UNKNOWN) and
IncidentFilterParamsSeverity (DOWN/DEGRADED), which causes oapi-codegen
to namespace-qualify previously-unique enum constants:

• generated.Email → generated.EmailChannelConfigChannelTypeEmail
• generated.Down → generated.TriggerRuleSeverityDown
• generated.Degraded → generated.TriggerRuleSeverityDegraded

It also makes managedBy nullable on CreateMonitorRequest (now
*pointer), and adds managedBy to Create/Update Status Page, Resource
Group, and Alert Channel DTOs.

Changes

• Refresh docs/openapi/monitoring-api.json from mono#369
• Regenerate internal/generated/types.go
• Update enum constant references to qualified names
• Wrap CreateMonitorRequest.ManagedBy in pointer (and similar for
  status page, resource group, alert channel) — provider continues to
  hardcode TERRAFORM for attribution so audit trail is correct
• Allow-list new managed_by fields in schema-vs-DTO audit
• Update monitor_test.go for pointer ManagedBy comparison

Why now

mono#369 Evolution Harness pulls TF provider source from main and
recompiles against the new spec. Without this PR, mono#369 cannot merge.
Cross-repo dependency: this PR must merge first.

Test plan

• go build ./... (clean locally)
• go test ./... (clean locally — all 4 packages pass)
• Existing acceptance tests should be unaffected (constant rename only,
  same wire format)

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