ci: add juju 4/beta to integration test matrix

[Copilot]

Issue

The _integration.yml workflow only tests with Juju 3.6/stable. We need coverage for 4/beta channel, but only for the tls_full scenario to avoid excessive test runtime.

Solution

Vectorized the matrix strategy to include juju-channel dimension:

• Added juju-channel: ["3.6/stable", "4/beta"] to matrix
• Used exclude to restrict 4/beta to tls_full scenario only
• Replaced hardcoded 3.6/stable references with ${{ matrix.juju-channel }}

Result: 5 test combinations (4 scenarios × 3.6/stable + 1 scenario × 4/beta) instead of 4.

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.

Context

GitHub Actions matrix exclusions allow selective test coverage without duplicating job definitions. The exclude key removes specific combinations from the cartesian product.

Testing Instructions

Trigger the workflow and verify:

• 5 jobs spawn (not 8)
• 4/beta runs only with tls_full
• 3.6/stable runs with all 4 scenarios

Upgrade Notes

No upgrade impact. This is a CI-only change.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• canonical-bos01.cdn.snapcraftcontent.com
  • Triggering command: /usr/lib/snapd/snapd /usr/lib/snapd/snapd (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

On _integration.yml, The _integration.yml workflow should have the matrix strategy expanded to vectorize "juju-channel". Currently we're only testing with "3.6/stable", and we should also run the same tests for channel "4/beta". However, for "4/beta" we only should run the "tls_full" variant.

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