Fix deprecation warnings

[ChrisRackauckas-Claude]

Summary

• Remove duplicate initial condition for PT2 test (pt2.xd already included in unknowns(sys) .=> 0.0)
• Remove bound parameter IC setting for PI/LimPI tests (gainPI.k is a bound parameter, can't set initial conditions on it)
• Replace deprecated @mtkbuild with @mtkcompile in analog electrical tests (4 occurrences)
• Fix deprecated integer indexing on DiffEqArray in hydraulic actuator test (initsol[1] → initsol.u[end]), and provide all unknowns instead of just first
• Remove unused deprecated imports (get_eqs, vars, @set!) in analysis points test
• Fix linearize call to not wrap scalar analysis point inputs/outputs in vectors

Test Results

With JULIA_DEPWARN=error:

• 577 passed, 0 failed, 6 errored, 3 broken
• All 3 deprecation-specific test failures (PT2, PI, LimPI) are fixed
• The 6 remaining errors are pre-existing upstream bugs in the Actuator System test (KeyError for port.ρ keys in initial_conditions dict) — these fail with or without JULIA_DEPWARN=error

Upstream Dependencies Needing Fixes

The following pre-existing issues are not caused by deprecation warnings but are blocking a fully clean test run:

1. ModelingToolkitBase (initial_conditions dict): Bound port parameters like src.port.ρ, valve.port_s.ρ, etc. are no longer accessible via initial_conditions(sys) dict indexing — causes 6 KeyErrors in the Actuator System test
2. SymbolicIndexingInterface: symbolic_container(::ModelingToolkitBase.IndexCache) method is missing — causes analysis points test failure (currently passing due to @test_broken or error being caught)
3. RecursiveArrayTools: Base.getindex(VA::AbstractDiffEqArray, i::Int) is deprecated in favor of VA.u[i] — fixed in this PR for the one occurrence in test code

Test plan

• Run JULIA_DEPWARN=error julia --project -e 'using Pkg; Pkg.test()'
• Verify PT2, PI, LimPI tests pass (previously failed with DEPWARN=error)
• Verify analysis points tests pass
• Verify electrical analog tests pass with @mtkcompile
• Run Runic.jl formatter

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