Fix bug in common path prefix calculation in Helidon generators

[tjquinno]

Resolves #20298

The Helidon generator contains code to calculate the longest common prefix among paths to help optimize the grouping of operations into services.

As described in the bug, the original implementation essentially compared character-by-character which led to problems if paths agreed partway through a segment. For example, given the paths "/a/part-one" and "/a/part-two" the code would incorrectly yield a common prefix of "a/part-" when it should have been "/a".

The PR revises the calculation so it examines corresponding segments rather than corresponding characters of the paths.

The PR includes additional tests to more thoroughly check the implementation.

(Also, the PR updates the hard-coded current latest releases of Helidon used by the generator if the user does not specify a Helidon version and if the available versions are not available on the network.)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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