Add Fleet 2026-02-01-preview API (includes MaxConcurrency)

[frantran]

ARM (Control Plane) API Specification Update Pull Request

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Purpose of this PR

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• New resource provider.
• New API version for an existing resource provider. (If API spec is not defined in TypeSpec, the PR should have been created in adherence to OpenAPI specs PR creation guidance).
• Update existing version for a new feature. (This is applicable only when you are revising a private preview API version.)
• Update existing version to fix OpenAPI spec quality issues in S360.
• Convert existing OpenAPI spec to TypeSpec spec (do not combine this with implementing changes for a new API version).
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  • edit this with your clarification

Due diligence checklist

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• I confirm this PR is modifying Azure Resource Manager (ARM) related specifications, and not data plane related specifications.
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  ARM resource provider contract and
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Additional information

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API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
TypeSpec	Microsoft.ContainerService
Go	sdk/resourcemanager/containerservicefleet/armcontainerservicefleet
JavaScript	@azure/arm-containerservicefleet
Python	azure-mgmt-containerservicefleet
Java	com.azure.resourcemanager:azure-resourcemanager-containerservicefleet