feat: Speed up custom-tool migration scans by limiting them to affected assemblies

[hatayama]

Summary

• Adds a way to scan and build a migration plan for only a specific set of assemblies, instead of always walking the whole project.

User Impact

• Currently, checking (or previewing) whether a V2-to-V3 custom-tool migration is needed always scans every file under Assets, even when only a couple of tools actually need it.
• This lays the groundwork for a much faster path: once the wizard already knows which files need migrating (from compile errors, added in a companion PR), it can rescan only the assemblies those files belong to instead of the entire project. This PR only adds the scoped scan/plan/preview machinery and its tests; it is not wired into the wizard's startup flow yet.

Changes

• ThirdPartyToolMigrationScanScopeResolver (new): resolves a set of seed file paths to their deduplicated assembly directories, reusing the existing FindNearestAssemblyDirectory resolution used by the full scan.
• ProjectFileInventory.CreateFromDirectoriesAsync (new): walks only the given scope directories instead of the whole Assets tree. Shares its directory-walk logic with the existing full-scan CreateAsync via an extracted WalkDirectoryTreeAsync helper — the full scan's behavior is unchanged.
• ThirdPartyToolMigrationPlanBuilder.CreateInScopeAsync / ThirdPartyToolMigrationFileService.PreviewMigrationInScopeAsync (new): distinctly-named scope-limited counterparts to the existing Create/CreateAsync/PreviewMigration(Async) methods (the repo forbids method overloading). The scoped preview intentionally does not read or write the existing full-scan preview/plan cache, since a partial-scope plan must never be mistaken for the full plan that ApplyMigration relies on.

Verification

• uloop compile: 0 errors, 0 warnings
• uloop run-tests (new + related tests, regex filter): 12/12 passed
• uloop run-tests (all ThirdPartyToolMigration* tests): 438/438 passed (4 skipped, platform-specific)
• uloop run-tests (full EditMode suite): 2309/2320 passed; the 4 failures are pre-existing and unrelated to this change (dispatcher version-pin string drift, a schema DescriptionAttribute check, a startup-hook ownership check) — none touch ThirdPartyToolMigration code.

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 5f470073-b0a4-4ac7-9344-d541bc1ddf32

📥 Commits

Reviewing files that changed from the base of the PR and between 5d59f98 and c622350.

⛔ Files ignored due to path filters (3)

• Assets/Tests/Editor/ThirdPartyToolMigrationScanScopeResolverTests.cs.meta is excluded by none and included by none
• Assets/Tests/Editor/ThirdPartyToolMigrationScopedPreviewTests.cs.meta is excluded by none and included by none
• Packages/src/Editor/Infrastructure/ThirdPartyToolMigration/ThirdPartyToolMigrationScanScopeResolver.cs.meta is excluded by none and included by none

📒 Files selected for processing (6)

• Assets/Tests/Editor/ThirdPartyToolMigrationScanScopeResolverTests.cs
• Assets/Tests/Editor/ThirdPartyToolMigrationScopedPreviewTests.cs
• Packages/src/Editor/Infrastructure/ThirdPartyToolMigration/ThirdPartyToolMigrationFileService.cs
• Packages/src/Editor/Infrastructure/ThirdPartyToolMigration/ThirdPartyToolMigrationPlanBuilder.cs
• Packages/src/Editor/Infrastructure/ThirdPartyToolMigration/ThirdPartyToolMigrationProjectFileInventory.cs
• Packages/src/Editor/Infrastructure/ThirdPartyToolMigration/ThirdPartyToolMigrationScanScopeResolver.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/assembly-scoped-migration-scan

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