Fix overdetermined initialization in Element1D and ConvectiveElement1D

[SebastianM-C]

In MTK v11, the syntax dT(t) = dT with a constant value creates an
initial_condition, which adds an initialization equation. For algebraic
variables like dT and Q_flow that are constrained by equations
(e.g., dT ~ port_a.T - port_b.T), this creates an overdetermined
initialization system.

The fix changes from initial conditions to guesses:

• Old: dT(t) = dT, [guess = 0.0] (dT arg as initial condition)
• New: dT(t), [guess = dT_guess] (dT_guess arg as guess only)

This follows the MTK v11 semantics where algebraic/observed variables
should only have guesses, not initial conditions.

Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.