Remove LinuxImageDemand and WindowsImageDemand variables from pipeline configuration

[Copilot]

Summary

This PR removes the LinuxImageDemand and WindowsImageDemand variables from all image.yml files and replaces their usage with hardcoded string values. This aligns with the changes made in Azure/azure-sdk-for-net#50802.

Changes Made

1. Removed variable definition: Deleted LinuxImageDemand variable from /eng/pipelines/templates/variables/image.yml
2. Replaced variable usage: Updated /eng/pipelines/docindex.yml to use the hardcoded value ImageOverride -equals ubuntu-24.04 instead of $(LinuxImageDemand)
3. Confirmed WindowsImageDemand: Verified that WindowsImageDemand was not present in the codebase

Before

# image.yml
- name: LinuxImageDemand
  value: ImageOverride -equals $(LINUXVMIMAGE)

# docindex.yml
pool:
  name: $(LINUXPOOL)
  demands: $(LinuxImageDemand)

After

# image.yml
# LinuxImageDemand variable removed

# docindex.yml
pool:
  name: $(LINUXPOOL)
  demands: ImageOverride -equals ubuntu-24.04

Validation

• ✅ YAML syntax validation passed for all modified files
• ✅ Confirmed no remaining references to either variable in the entire codebase
• ✅ Changes are minimal and surgical (2 files, 4 lines changed)
• ✅ Hardcoded value correctly resolves the original variable expression

The replacement value ImageOverride -equals ubuntu-24.04 was determined by resolving the original variable definition: LinuxImageDemand = "ImageOverride -equals $(LINUXVMIMAGE)" where LINUXVMIMAGE = "ubuntu-24.04".

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[Copilot]

Pull Request Overview

This PR removes the intermediary LinuxImageDemand variable from the pipeline configuration and replaces its usage with a hardcoded string value to align with changes in the Azure/azure-sdk-for-net repository.

• Removed the LinuxImageDemand variable definition from eng/pipelines/templates/variables/image.yml.
• Updated eng/pipelines/docindex.yml to use the hardcoded value "ImageOverride -equals ubuntu-24.04" in place of the variable.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
eng/pipelines/templates/variables/image.yml	Removed the LinuxImageDemand variable to eliminate unnecessary indirection.
eng/pipelines/docindex.yml	Replaced variable reference with a hardcoded string value.

Comments suppressed due to low confidence (2)

eng/pipelines/docindex.yml:31

• [nitpick] Consider adding an inline comment explaining why the literal value is used instead of a variable to improve future clarity and maintainability.

      demands: ImageOverride -equals ubuntu-24.04