feat(terraform): add volume backup exclusion resource (EON-7912)

[atlowChemi]

Summary

Companion PR to eon-io/eon-service#28165. Adds a new eon_volume_backup_exclusion Terraform resource that allows excluding specific EBS volumes from future EC2 instance backups.

• Client layer: Two new direct-HTTP methods (ExcludeVolumeFromBackup, CancelVolumeBackupExclusion) calling PATCH /v1/projects/{projectId}/resources/{id}/volumes/{volumeId}/exclude and .../include. Uses raw HTTP (same pattern as StartBigQueryDatasetRestore) because the SDK doesn't have these endpoints yet.
• Resource: eon_volume_backup_exclusion — create excludes, destroy re-includes. Both attributes (resource_id, volume_id) require replacement on change. Supports import via resource_id/volume_id format.
• Read is a no-op: There is no GET endpoint to check exclusion status, so Read just persists state. No drift detection.
• Docs: Auto-generated via go generate ./... using terraform-plugin-docs.

Review & Testing Checklist for Human

• Verify API paths match eon-service PR: Confirm the URLs (/resources/{id}/volumes/{volumeId}/exclude and /include) match the OpenAPI spec in eon-service#28165 exactly — a mismatch here would cause silent 404s at runtime.
• Accept the no-op Read: The resource cannot detect drift — if a volume is re-included outside Terraform, state won't update. Decide if this is acceptable or if a read-back mechanism is needed.
• Accept import without validation: ImportState sets state without calling any API. An import of a non-existent or non-excluded volume will succeed silently.
• Decide if unit tests are needed: No unit tests were added for the new resource or client methods. Other resources in this repo have mock-based tests.

Suggested test plan: After the eon-service PR is merged, apply the resource against a real EC2 instance with multiple volumes, verify the non-root volume becomes excluded, then terraform destroy and verify it's re-included.

Notes

• The golangci-lint config needs a version field for golangci-lint v2 — this is a pre-existing issue, not introduced by this PR. go vet and go test both pass cleanly.
• Mock client stubs always return nil (no error tracking), consistent with existing mock patterns.

Link to Devin session: https://eon.devinenterprise.com/sessions/0a6eebe354004283bfc89ebb9daee533
Requested by: @atlowChemi

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