Remove LinuxImageDemand and WindowsImageDemand variables, replace hardcoded ubuntu-24.04 with $(LINUXVMIMAGE)

[Copilot]

Summary

This PR aligns the Azure SDK for Python repository with the infrastructure changes made in Azure/azure-sdk-for-net#50802 by removing deprecated image demand variables and standardizing on centralized image variable references.

Changes Made

1. Variable Removal

• Removed LinuxImageDemand variable definition from /eng/pipelines/templates/variables/image.yml
• No WindowsImageDemand variable was found in the codebase (already compliant)

2. Usage Pattern Updates

• Updated docindex.yml to use demands: ImageOverride -equals $(LINUXVMIMAGE) instead of demands: $(LinuxImageDemand)

3. Hardcoded Value Standardization

Replaced all hardcoded ubuntu-24.04 references with $(LINUXVMIMAGE) variable in 14 pipeline files:

• eng/pipelines/tsp-spec-sync.yml
• eng/pipelines/trigger-ml-sample-pipeline.yml
• eng/pipelines/generate-all-docs.yml
• eng/pipelines/templates/stages/archetype-sdk-tests.yml
• eng/pipelines/templates/stages/archetype-conda-release.yml
• eng/pipelines/templates/stages/archetype-python-release.yml (7 occurrences)
• eng/pipelines/templates/stages/partner-release.yml
• eng/pipelines/templates/stages/conda-sdk-client.yml
• eng/pipelines/templates/jobs/run-cli-tests.yml
• eng/pipelines/templates/jobs/build-conda-dependencies.yml
• eng/pipelines/templates/jobs/ci.yml (2 occurrences)
• eng/pipelines/templates/jobs/tests-nightly-python.yml (2 occurrences)

Before and After Examples

Before:

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

# In pipeline files
demands: $(LinuxImageDemand)
image: ubuntu-24.04

After:

# LinuxImageDemand variable removed from image.yml

# In pipeline files  
demands: ImageOverride -equals $(LINUXVMIMAGE)
image: $(LINUXVMIMAGE)

Benefits

• 🎯 Consistency: Aligns with latest Azure SDK infrastructure requirements across all language repositories
• 🔧 Maintainability: Centralized image version management through $(LINUXVMIMAGE) variable
• 🚀 Flexibility: Ubuntu version updates now require changing only one variable definition instead of multiple hardcoded values
• 📋 Standardization: Follows the established pattern from the .NET repository changes

Validation

• ✅ All YAML files validated for syntax correctness
• ✅ No remaining LinuxImageDemand or WindowsImageDemand references in codebase
• ✅ All $(LINUXVMIMAGE) variable references use proper Azure DevOps syntax
• ✅ The LINUXVMIMAGE variable remains properly defined as ubuntu-24.04 in image.yml

This change ensures the Python SDK repository infrastructure remains aligned with the broader Azure SDK ecosystem while maintaining backward compatibility and improving maintainability.

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