Make sure fixed curvature lookahead distance doesn't overshoot distance to cusp with RPP controller (backport #5134)

[mergify]

Related issue: #5098

---

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#5098
Primary OS tested on	Ubuntu
Robotic platform tested on	Loopback Simulation
Does this PR contain AI generated software?	No

---

Description of contribution in a few bullet points

Limit fixed curvature lookahead distance to distance to cusp if the latter is smaller.

Is only relevant if both allow_reversing and use_fixed_curvature_lookahead is true, as otherwise there will be no cusps in the path, or lookahead will automatically be adjusted correctly already.

Description of documentation updates required from your changes

Bugfix: no change required.

Description of how this change was tested

Test run in TB4 Loopback Simulation.

Future work that may be required in bullet points

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

---

This is an automatic backport of pull request #5134 done by [Mergify](https://mergify.com).

[mergify]

@mergify[bot], all pull requests must be targeted towards the main development branch.
Once merged into main, it is possible to backport to @jazzy, but it must be in main
to have these changes reflected into new distributions.