[DdoS]Add DDoS custom policy cmdlets, LB integration, tests, and help

[venkatasaimaram7]

## Description

This PR added DDoS Custom Policy support in Az.Network, including new cmdlets, load balancer frontend integration, model/mapping updates, help docs, and scenario test coverage.

Changes included:

• Added new cmdlets:
  • Get-AzDdosCustomPolicy
  • New-AzDdosCustomPolicy
  • Remove-AzDdosCustomPolicy
• Added DdosCustomPolicyId support in load balancer frontend IP configuration cmdlets:
  • Add-AzLoadBalancerFrontendIpConfig
  • New-AzLoadBalancerFrontendIpConfig
  • Set-AzLoadBalancerFrontendIpConfig
• Added and updated Network models/mappings for DDoS custom policy and LB frontend DDoS settings.
• Added scenario tests and session recordings for:
  • DDoS custom policy CRUD
  • DDoS custom policy with tags
  • LB frontend attach/detach with DDoS custom policy
• Added markdown help files for the new cmdlets.
• Updated service changelog in ChangeLog.md.

Cmdlet design review:

• Add approved design review link here (if required by process).

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • General release
  • Public preview
  • Private preview
  • Engineering build
  • No need for a release

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request

## Validation

• Build succeeded locally.
• DdosCustomPolicyTests playback passed: 4 passed, 0 failed.
• End-to-end validation completed for DCP attach/detach with LB FIP.
  • Verified attach flow and reflected references in cmdlet outputs.
  • Verified detach flow and removal of association from LB frontend.
    - Validation artifact:
  • https://microsoft-my.sharepoint.com/:w:/p/venm/IQDgU4fOroEwR5PF6Ib8iDiJASg0iRGXgwag7sV35-uK2Fk?e=YDIc7H
• StaticAnalysis command failed in this environment with analyzer host exit code -532462766 (tool/runtime issue).

[VeryEarly]

/azp run

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