Migrate to CTBase 0.28 parameter contract

[ocots]

Summary

This PR migrates all CTSolvers strategies to the breaking changes introduced in CTBase.jl v0.28.0-beta.

Closes #171.

Changes

Source code (8 strategy files)

• Renamed Strategies._default_parameter → Strategies.default_parameter in all 7 parameterized strategies (ADNLP, Exa, Ipopt, Knitro, MadNLP, MadNCL, Uno) and all their call sites (constructors, metadata fallbacks).
• Added Strategies.parameter(::Type{<:S{P}}) where {P<:Bound} = P for each of the 7 parameterized strategies, with the appropriate type bound (CPU for CPU-only, Union{CPU,GPU} for GPU-capable).
• Added Strategies.parameter(::Type{<:SciML}) = nothing for the non-parameterized SciML integrator.
• Added docstrings for every parameter method, following the Handbook docstring templates ($(TYPEDSIGNATURES), one-sentence summary, # Returns, See also: with cross-references).

Tests (11 files)

• Replaced all _default_parameter calls with default_parameter.
• Replaced all get_parameter_type calls with parameter.
• Added parameter (and default_parameter where needed) to fake test strategies in test_backward_compatibility.jl, test_integration_parameters.jl, test_alias_integration.jl, and test_mode_propagation.jl.
• Updated test_parameter_contract.jl to test the parameter fallback (in addition to default_parameter), with assertions matching CTBase's actual error message format.
• Updated commented-out type-stability tests to use the new method names.

Documentation (3 guide files)

• implementing_a_solver.md: Updated contract list, code examples, and summary checklist to reference parameter and default_parameter instead of _default_parameter.
• implementing_a_modeler.md: Same updates, including Exa example with Union{CPU,GPU} parameter method.
• architecture.md: Updated tag-dispatch example and parameter resolution description.

Historical files

• BREAKING.md: Rewrote v0.4.0-beta entries to use parameter/default_parameter instead of _supported_parameters/_default_parameter.
• CHANGELOG.md: Same updates for the v0.4.0-beta section.

Project.toml

• Bumped CTBase compat from 0.26, 0.27 to 0.28.
• Bumped CTModels compat from 0.14, 0.15 to 0.15.
• Bumped version to 0.4.27-beta.
• Updated docs/Project.toml CTBase compat accordingly.

Verification

All 393 strategy tests pass:

Test Summary:                                              | Pass  Total  Time
CTSolvers tests                                            |  393    393  30.8s

[ocots]

Closes #171.