Remove orphaned vfs.Overlay in-memory filesystem

[denik]

vfs.Overlay returned a Path that served in-memory files on top of a real tree. It was added in #6110 (Aug 2026) to inject generated files into the bundle sync root for AI Runtime code_source_path.

Its only caller was removed in #6494 (Sep 2026), which packages the code_source as a tgz artifact instead of a sync overlay. Since then Overlay (and its whole overlayPath/memFile machinery) has been reachable only from its own unit tests.

deadcode -test ./... doesn't catch this because the test suite keeps it "live"; it surfaces only when reachability is computed without test roots.

This pull request and its description were written by Isaac.

[eng-dev-ecosystem-bot]

Integration test report

Commit: 1bec84e

Run: 34607707486

	Env	🔄​flaky	💚​RECOVERED	✅​pass	🙈​skip	Time
💚​	aws linux		1	275	15	5:31
💚​	aws windows		1	277	13	4:54
💚​	azure linux		1	274	15	5:50
🔄​	azure windows	14	1	262	13	12:44
💚​	gcp linux		1	275	15	6:24
💚​	gcp windows		1	277	13	5:40

15 interesting tests: 14 flaky, 1 RECOVERED

	Test Name	aws linux	aws windows	azure linux	azure windows	gcp linux	gcp windows
💚​	TestAccept	💚​R	💚​R	💚​R	💚​R	💚​R	💚​R
🔄​	TestFsCatOnNonExistentFile	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCatOnNonExistentFile/dbfs	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpDir/dbfs_to_dbfs	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpDir/dbfs_to_local	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpDir/dbfs_to_uc-volumes	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpDir/uc-volumes_to_dbfs	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpDir/uc-volumes_to_uc-volumes	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpFileToFile	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestFsCpFileToFile/local_to_dbfs	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestLock	✅​p	✅​p	✅​p	🔄​f	✅​p	✅​p
🔄​	TestTagKeyAzure	🙈​s	🙈​s	✅​p	🔄​f	🙈​s	🙈​s
🔄​	TestTagKeyAzure/valid			✅​p	🔄​f
🔄​	TestTagValueAzure	🙈​s	🙈​s	✅​p	🔄​f	🙈​s	🙈​s
🔄​	TestTagValueAzure/valid			✅​p	🔄​f

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
3:28	aws windows	TestAccept
3:27	gcp windows	TestAccept
3:20	azure windows	TestAccept