Wire observation_field test into Core

[ChrisRackauckas-Claude]

Summary

PR #543 added test/bodies/observation_field.jl but never added the top-level @safetestset wrapper that SciMLTesting's folder-discovery mode runs under GROUP=Core (test/bodies/ is explicitly non-discovered). This adds test/observation_field.jl in the same pattern as the other Core siblings so the existing body is actually executed.

Verification

Isolated body (after wiring wrapper pattern / proper test deps):

julia --project=scratch/pkgtest_env -e 'using SafeTestsets; @safetestset "Observation field" begin include(".../test/bodies/observation_field.jl") end'

Tail:

Test Summary:     | Pass  Total     Time
Observation field |    1      1  2m20.9s

Core group:

GROUP=Core julia --project=. -e 'using Pkg; Pkg.test()'

Tail (includes newly discovered file; full suite green):

Core/observation_field.jl |    1      1  1.3s
...
Core/y_saturation.jl |    9      9  4m33.9s
     Testing StructuralIdentifiability tests passed

Runic --inplace and typos on test/observation_field.jl: clean.

Not verified

QA / ModelingToolkitSIExt groups; package docs build; GitHub CI (master workflow ref still broken pending #561). Did not touch .github/.

Please ignore this draft until reviewed by @ChrisRackauckas.

Risk assessment

• Risk: Low — one new Core @safetestset wrapper including an already-merged body; no production code changes.
• Blast radius: Core test discovery only; adds ~1–2 minutes of previously-unrun compute to Core.
• Evidence: Isolated body 1/1 pass; full GROUP=Core Pkg.test() passed with Core/observation_field.jl in the summary.
• Independent review: pending
• Merge: needs review

Harness: Cursor Agent CLI 2026.10.01-14929f9; model: auto; transcript: /home/crackauc/sandbox/goals/qa-hygiene/jobs/si-obsfield/log.txt

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