Add test coverage for git sources with transitive dependency fallback

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

The original PR #9301 fixed fallback behavior for both Path and Git sources when transitive dependencies don't exist in the source, but only included test coverage for Path sources. Git sources inherit from Path, so the fix applies to both, but lacked regression protection.

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

Added parallel test case for git sources mirroring the existing path source test:

context "when a git gem has a transitive dependency that does not exist in the git source" do
  # Tests that missing_dep falls back to rubygems when parent_gem is git-sourced
  it "falls back to the default rubygems source for that dependency when updating"
end

Both path and git source tests verify the fix at bundler/lib/bundler/definition.rb:1080 works correctly.

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

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[Copilot]

Pull request overview

Adds missing regression coverage to ensure Bundler’s transitive dependency fallback behavior works for git-sourced gems (in addition to the existing path-sourced coverage), matching the fix introduced in PR #9301.

Changes:

• Adds a new spec context for a git gem whose transitive dependency is not present in the git source.
• Verifies that during bundle update, the missing transitive dependency continues to resolve from the default RubyGems source while the parent remains git-sourced.

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