fix: serialize concurrent marketplace repo clones via SequencerByKey

[Copilot]

Adding a new plugin marketplace could trigger concurrent ensureRepository calls for the same repo. Both would see the directory missing, both would attempt git clone, and the second would fail with fatal: could not create work tree dir '...': File exists.

• AgentPluginRepositoryService: add _cloneSequencer: SequencerByKey<string> and wrap ensureRepository body in _cloneSequencer.queue(marketplace.canonicalId, ...). Concurrent calls for the same canonical ID are serialized — the second waits for the first, then finds the directory already exists and returns early.
• Test: add a test that fires two concurrent ensureRepository calls for the same marketplace and asserts _git.cloneRepository is invoked exactly once.

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[Copilot]

Pull request overview

Prevents race conditions when multiple callers attempt to clone the same plugin marketplace repository concurrently by serializing ensureRepository work, and adds a regression test around concurrent calls.

Changes:

• Add a SequencerByKey<string> to serialize concurrent ensureRepository calls.
• Wrap ensureRepository clone flow inside the sequencer queue.
• Add a browser test that fires concurrent ensureRepository calls and asserts clone happens once.

Reviewed changes

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

File	Description
src/vs/workbench/contrib/chat/browser/agentPluginRepositoryService.ts	Serializes ensureRepository to avoid concurrent git clone into the same target directory.
src/vs/workbench/contrib/chat/test/browser/plugins/agentPluginRepositoryService.test.ts	Adds a concurrent-call test asserting _git.cloneRepository is invoked only once.

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