Fixes for nav2_smac_planner to build with -Werror.

[clalancette]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Follow-up to #4634
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

This is a follow-up PR to #4634 where we change nav2_smac_planner to use nav2_package. This turns on additional compiler warnings, along with -Werror, so it requires some fixes to go along with it.

I want to highlight one controversial change in here. The UNKNOWN constant in constants.hpp was causing shadowing problems with UNKNOWN embedded inside of enum class elsewhere in the code. So I renamed that constant (along with the rest) to UNKNOWN_COST. This breaks the API, but resolves the error.

Description of documentation updates required from your changes

It will possibly be necessary to update the documentation to mention the API break in constants.hpp.

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Future work that may be required in bullet points

None needed.

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Files	Patch %	Lines
nav2_smac_planner/src/node_hybrid.cpp	92.30%	1 Missing ⚠️

Files	Coverage Δ
...ac_planner/include/nav2_smac_planner/constants.hpp	100.00% <ø> (ø)
...c_planner/include/nav2_smac_planner/node_basic.hpp	100.00% <100.00%> (ø)
...2_smac_planner/include/nav2_smac_planner/types.hpp	100.00% <ø> (ø)
nav2_smac_planner/src/analytic_expansion.cpp	86.15% <ø> (ø)
nav2_smac_planner/src/collision_checker.cpp	96.87% <100.00%> (ø)
nav2_smac_planner/src/costmap_downsampler.cpp	93.54% <100.00%> (ø)
nav2_smac_planner/src/node_2d.cpp	98.03% <100.00%> (ø)
nav2_smac_planner/src/smac_planner_hybrid.cpp	86.84% <ø> (+0.22%)	⬆️
nav2_smac_planner/src/smoother.cpp	94.50% <100.00%> (ø)
nav2_smac_planner/src/node_hybrid.cpp	87.59% <92.30%> (ø)

... and 2 files with indirect coverage changes

[SteveMacenski]

Christ. This kind of amateur shit I have no excuse for. I don't know how nav2_package() got overlooked for this package. A few of these probably created real bugs. This was a really good nitpick note to uncover. Thanks Chris!