Bulk dependency updates (ready checks)

[AArnott]

Summary

This PR aggregates dependency update branches that are ready to merge (all non-docfx checks passing):

• Update dependency dotnet-coverage to v18.8.0 - autoclosed #1437 Update dependency dotnet-coverage to v18.7.0
• Update dependency powershell to v7.6.2 - autoclosed #1440 Update dependency powershell to v7.6.2
• Update actions/checkout action to v6.0.3 #1449 Update actions/checkout action to v6.0.3

Validation

• Ran ./.github/Prime-ForCopilot.ps1
• Ran dotnet build StreamJsonRpc.slnx -c Release (succeeded)
• Ran dotnet test --no-build -c Release -- --filter-not-trait "FailsInCloudTest=true"
  • Several existing/flaky tests failed in StreamJsonRpc.Tests (memory leak and PolyType-related assertions).
  • Individual dependency PR checks are already green for build/test; only docfx is failing on Update dependency dotnet-coverage to v18.8.0 - autoclosed #1437/Update dependency powershell to v7.6.2 - autoclosed #1440 and is considered non-blocking per dependency-bundling instructions.

[Copilot]

Pull request overview

This PR bundles a set of “ready” dependency updates by bumping pinned tool/action versions used in CI and developer tooling, keeping the repo’s automation up-to-date while maintaining commit-SHA pinning for GitHub Actions.

Changes:

• Bump actions/checkout from v6.0.2 to v6.0.3 (pinned by commit SHA) across all workflows that use it.
• Update .NET local tools: powershell 7.6.1 → 7.6.2 and dotnet-coverage 18.6.2 → 18.7.0.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
.github/workflows/libtemplate-update.yml	Updates pinned actions/checkout SHA/comment to v6.0.3 for the template-merge automation.
.github/workflows/docs.yml	Updates pinned actions/checkout SHA/comment to v6.0.3 for the docs publish workflow.
.github/workflows/docs_validate.yml	Updates pinned actions/checkout SHA/comment to v6.0.3 for doc validation.
.github/workflows/copilot-setup-steps.yml	Updates pinned actions/checkout SHA/comment to v6.0.3 for Copilot setup steps.
.config/dotnet-tools.json	Updates tool versions for powershell and dotnet-coverage.