Fix joint downgrade test resolution

[ChrisRackauckas-Claude]

Important

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

• resolve the root and test projects together in the downgrade job
• run the package's Core test group explicitly
• restore push-trigger coverage for the repository's main branch

Root cause

The previous workflow downgraded only the root project. Pkg.test then activated the separate, pre-existing test/Project.toml, after the root manifest had already selected LinearSolve 5. That could not resolve with the minimum OrdinaryDiffEq 7 / OrdinaryDiffEqDefault 2 graph in the test environment.

The deployed downgrade action supports joint project resolution. Passing projects: ".,test" lets the resolver choose one compatible graph before Pkg.test; the final locked graph contains LinearSolve 4.3.0, OrdinaryDiffEq 7.1.3, and OrdinaryDiffEqDefault 2.3.0.

The workflow also watched master, but this repository's default branch is main, and it did not pass a test group to the SciMLTesting harness.

Local verification

Julia 1.10.11 with the deployed v2.6.1 downgrade action in deps mode, resolving .,test, followed by a strict locked GROUP=Core test run:

Test Summary:         | Pass  Total      Time
Core/DataReduction.jl |   15     15  17m46.0s
Test Summary: | Pass  Total     Time
Core/deim.jl  |    4      4  5m12.0s
Test Summary: | Pass  Total  Time
Core/utils.jl |   10     10  2.0s
     Testing ModelOrderReduction tests passed

Additional checks passed:

• Pkg.build() on the exact locked graph
• Runic --check .
• actionlint .github/workflows/Downgrade.yml
• Project TOML parse
• git diff --check

[ChrisRackauckas-Claude]

CI timer result:

• Downgrade, canonical Runic, documentation, Core on Julia LTS/current/pre, and QA are all green.
• The only red check is Runic Suggestions. Its formatting step succeeded; the later reviewdog posting step failed while fetching repository metadata with GitHub API HTTP 503.
• A job-only retry through the Actions API returned HTTP 403 because repository-admin rights are required.

This is an external suggestion-posting failure, not a formatting or functional failure. No source change is indicated.

[ChrisRackauckas-Claude]

Verified the fork head still matched cb303cc exactly and the branch is current with main. Runic passed locally, then I pushed empty co-authored retry commit 71f8543 to rerun runic-suggestions after its reviewdog GitHub API 503; the retry commit has no tree changes.