Add URINormalizer trailing slash specs

[afurm]

What was the end-user or developer problem that led to this PR?

The trailing-slash normalization behavior in Bundler affects lockfile expectations (gem sources keep a trailing slash, git sources do not), but there were no focused specs to lock that behavior. This made future normalization changes risky and prone to unintended lockfile diffs.

What is your fix for the problem, implemented in this PR?

Add a focused spec for Bundler::URINormalizer.normalize_suffix covering both trailing_slash: true and false with inputs that do and do not already include a trailing slash. This locks the current behavior and makes future refactors safer.

Tests:

• cd bundler && bin/rspec spec/bundler/uri_normalizer_spec.rb (fails locally: missing rubygems-generate_index gem)

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[hsbt]

👍