Add plugin hooks around gem fetch and git source fetch

Adds four new hook points:
- before-fetch / after-fetch: fires in Source::Rubygems#download_gem
  around actual network downloads, avoiding noise from cache hits.
- before-git-fetch / after-git-fetch: fires in Source::Git#specs
  around fetch/checkout operations.

Based on the original proposal in #8162 with adjustments:
- Moved gem fetch hooks from fetch_gem_if_possible to download_gem
  so they only fire on actual network I/O.
- Dropped the source argument since spec.source provides it.
- Renamed git hooks to before-git-fetch / after-git-fetch for
  consistency with the existing before-*/after-* pattern.
- Removed GEM_BEFORE_FETCH/GEM_AFTER_FETCH from Source::Git#install
  since using gem fetch events for git sources is semantically
  inconsistent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

bundler/lib/bundler/plugin/events.rb

@@ -45,6 +45,30 @@ def self.defined_event?(event)
       #   GEM_AFTER_INSTALL = "after-install"
       define :GEM_AFTER_INSTALL,  "after-install"
 
+      # @!parse
+      #   A hook called before each individual gem is downloaded from a remote source.
+      #   Includes a Gem::Specification. Does not fire on cache hits.
+      #   GEM_BEFORE_FETCH = "before-fetch"
+      define :GEM_BEFORE_FETCH, "before-fetch"
+
+      # @!parse
+      #   A hook called after each individual gem is downloaded from a remote source.
+      #   Includes a Gem::Specification. Does not fire on cache hits.
+      #   GEM_AFTER_FETCH = "after-fetch"
+      define :GEM_AFTER_FETCH, "after-fetch"
+
+      # @!parse
+      #   A hook called before a git source is fetched.
+      #   Includes a Bundler::Source::Git reference.
+      #   GIT_BEFORE_FETCH = "before-git-fetch"
+      define :GIT_BEFORE_FETCH, "before-git-fetch"
+
+      # @!parse
+      #   A hook called after a git source is fetched.
+      #   Includes a Bundler::Source::Git reference.
+      #   GIT_AFTER_FETCH = "after-git-fetch"
+      define :GIT_AFTER_FETCH, "after-git-fetch"
+
       # @!parse
       #   A hook called before any gems install
       #   Includes an Array of Bundler::Dependency objects

bundler/lib/bundler/source/git.rb

@@ -191,8 +191,10 @@ def specs(*)
         set_cache_path!(app_cache_path) if use_app_cache?
 
         if requires_checkout? && !@copied
+          Plugin.hook(Plugin::Events::GIT_BEFORE_FETCH, self)
           fetch unless use_app_cache?
           checkout
+          Plugin.hook(Plugin::Events::GIT_AFTER_FETCH, self)
         end
 
         local_specs

bundler/lib/bundler/source/rubygems.rb

@@ -477,9 +477,11 @@ def download_gem(spec, download_cache_path, previous_spec = nil)
         Bundler.ui.confirm("Fetching #{version_message(spec, previous_spec)}")
         gem_remote_fetcher = remote_fetchers.fetch(spec.remote).gem_remote_fetcher
 
+        Plugin.hook(Plugin::Events::GEM_BEFORE_FETCH, spec)
         Gem.time("Downloaded #{spec.name} in", 0, true) do
           Bundler.rubygems.download_gem(spec, uri, download_cache_path, gem_remote_fetcher)
         end
+        Plugin.hook(Plugin::Events::GEM_AFTER_FETCH, spec)
       end
 
       # Returns the global cache path of the calling Rubygems::Source object.