Raise ModelingToolkitBase DelayDiffEq floor to 6.1.2

[ChrisRackauckas-Claude]

What changed

Raise ModelingToolkitBase’s minimum DelayDiffEq version from 6.1 to 6.1.2. The preceding floor was merged in #4994, but its hosted sublibrary DowngradeCI still resolved DelayDiffEq 6.1.0 and reproduced nine OrdinaryDiffEqCore.DEOptions constructor errors. 6.1.2 is the first registered release that passes the exact minimum stack. This is a dependency-compatibility change only: no public API changed and no deprecation path is required.

Verification

• Hosted failure on the merged 6.1 floor: https://github.com/SciML/ModelingToolkit.jl/actions/runs/32356973039/job/96388516017 — InitializationSystem Test | 605 passed, 9 errors, 12 broken; the log confirms DelayDiffEq v6.1.0.
• Exact minimum resolver with DelayDiffEq 6.1.2: Successfully resolved minimal versions for lib/ModelingToolkitBase (with extras).
• Local exact minimum GROUP=Initialization test: InitializationSystem Test | 660 pass, 12 broken, 672 total; Initial Values Test | 65 pass, 65 total; Testing ModelingToolkitBase tests passed.
• The resolved local stack contains DelayDiffEq v6.1.2, OrdinaryDiffEqCore v4.14.3, and SciMLBase v3.48.0.
• git diff --check passed and the final PR diff is one compat line.

The hosted Typos check reports pre-existing errors in .github and docs/src/internals/mtkcompile.md; this PR does not touch those files. GPU and downstream jobs were not run locally.

Please ignore this PR until it has been reviewed by @ChrisRackauckas.