Fix/copilot setup steps shares steps with lint and test

[Flix6x]

Description

• Fix copilot-setup-steps.yml, which was giving errors
• Added changelog item in documentation/changelog.rst

Look & Feel

This PR aims to make the test environment used by agents and by the lint-and-test workflow identical.

How to test

• lint-and-test should run upon creating this PR.
• I'm not sure if copilot uses the new copilot-setup-steps.yml until it is on main.

[read-the-docs-community]

Documentation build overview

📚 flexmeasures | 🛠️ Build #31656005 | 📁 Comparing bca347f against latest (eb8c218)

🔍 Preview build

Show files changed (2 files in total): 📝 2 modified | ➕ 0 added | ➖ 0 deleted

File	Status
changelog.html	📝 modified
api/v3_0.html	📝 modified

[Copilot]

Pull request overview

This PR attempts to unify the Python/PostgreSQL test environment setup between the Copilot agent setup workflow and the lint-and-test CI workflow, so both use the same installation steps.

Changes:

• Introduces a shared “setup test environment” definition intended to be reused by multiple workflows.
• Refactors lint-and-test and copilot-setup-steps to call that shared setup instead of duplicating steps.
• Aligns test environment variables/healthcheck formatting for the PostgreSQL service.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
.github/workflows/setup-test-env.yml	Adds a new shared setup definition (currently authored as a composite action).
.github/workflows/lint-and-test.yml	Switches test job to call the shared setup definition; updates cache key and env vars.
.github/workflows/copilot-setup-steps.yml	Switches Copilot setup workflow to call the shared setup definition; adds postgres/redis services and env vars.

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[Flix6x]

@copilot the workflow now fails on setting up the test environment with Error: Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/home/runner/work/flexmeasures/flexmeasures/.github/actions/setup-test-env'. Did you forget to run actions/checkout before running your local action? Please fix this by committing to this PR branch. Should the reference mention setup-test-env.yml instead of setup-test-env? Finally, verify whether using the copilot-setup-steps.yml (in this branch!) works.

[Copilot]

@Flix6x I've opened a new pull request, #1999, to work on those changes. Once the pull request is ready, I'll request review from you.