Fix LinearRepnVisitor to substitute fixed vars already in var_map

[cjck944084735-dot]

Fixes #3851 .

Summary/Motivation:

LinearRepnVisitor.record_monomial only applied the fixed-variable (constant) handling when the variable had not yet been registered in the visitor's var_map. When a variable was registered in bulk by the TemplateVarRecorder while it was still free and was fixed afterwards, subsequent walk_expression calls found it in the var_map and treated it as an unrestricted variable: fixed values were not substituted and expressions involving them were misclassified as nonlinear. This is exactly the failure mode reported in #3851, including through the LinearStandardFormCompiler's use of a TemplateVarRecorder.

Changes proposed in this PR:

• record_monomial now checks var.fixed before the var_map membership test, so a fixed variable always contributes its value as a constant, regardless of when it was registered.
• Two regression tests in pyomo/repn/tests/test_linear.py::TestFixedVarAfterRegistration, using the reporter's scenario (a variable registered by walking one expression while free, fixed, then used in another expression):
  • test_fixed_var_after_registration: x[1] - 1/x[2] with x[2].fix(2) now yields constant=-0.5, linear={x[1]: 1}, nonlinear=None;
  • test_fixed_var_substituted_before_registration: the fixed variable never becomes a linear term nor leaks into the nonlinear remainder.

After the change, the reporter's r2 is LinearRepn(mult=1, const=-0.5, linear={x[1]: 1}, nonlinear=None) instead of carrying nonlinear=-(1/x[2]).

Local verification

• pyomo/repn/tests/: 357 passed (355 existing + 2 new), 48 skipped.
• Broader pyomo/core/tests/: 4392 passed — no regressions.
• black --check clean on the changed files.

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Notes for reviewers (optional): The behavior change is narrow: a fixed variable that was registered in var_map while free is now handled as a constant. Callers that relied on such variables appearing as linear terms would have received incorrect coefficients (the fixed value was never substituted), so this should be strictly a correctness improvement. One thing I would appreciate a second look at: whether any writer depends on fixed-but-registered variables still occupying a column in var_map/var_list.

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