Fallback git/path sources to default source

[hsbt]

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

We've added a feature that uses git source to resolve bundler dependencies, including activerecord, on which Rails depends, in git monorepo such as rails/rails.

However, since rails dependencies, such as rack, are not in the rails monorepo, it is appropriate to use the default source, rubygems.org, for such gems.

Fixes #9258

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

The if statement on line 1080 now uses parent_dep.source only if the source indicated by parent_dep has target gemspec.

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

[Copilot]

Pull request overview

This PR fixes issue #9258 where Bundler 4.0.4 incorrectly attempts to resolve transitive dependencies from path/git sources instead of falling back to the default RubyGems source when those dependencies don't exist in the path/git source.

Changes:

• Adds logic to verify that a spec actually exists in a path/git source before using that source for dependency resolution
• Includes a test case demonstrating the fix for path sources
• Adds a version constraint to diff-lcs dependency in test setup

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
bundler/lib/bundler/definition.rb	Adds check to verify spec exists in path/git source before using it for transitive dependencies
bundler/spec/install/gemfile/sources_spec.rb	Adds test case for path gem with transitive dependency that should fall back to default source
bundler/spec/spec_helper.rb	Adds version constraint to diff-lcs dependency (appears unrelated to main fix)

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[Copilot]

@hsbt I've opened a new pull request, #9303, to work on those changes. Once the pull request is ready, I'll request review from you.