[api] Move requestFileSystem into project - #64285

Moves request filesystem layering into the project snapshot system so API-provided files correctly override editor overlays.

Changes:

• Integrates request layers with snapshot, directory, and auto-import file access.
• Moves request filesystem types and change tracking into project.
• Expands API and filesystem-layer tests.

• Files reviewed: 26/26 changed files
• Comments generated: 4
• Review effort level: Balanced

Suppressed comments (3)

tsc/internal/project/autoimport.go:52

• The auto-import implementation also reads the host before checking the shared request-handle cache, which causes every repeated lookup through a host symlink to hit the underlying filesystem. Check requestFileHandles first so this path has the same caching behavior as the other request-layer lookups.

		if lookup.host {
			if content, ok := layer.base.ReadFile(lookup.path); ok {
				return a.snapshotFSBuilder.requestFileHandle(originalFileName, originalPath, content)
			}

tsc/internal/project/snapshotfs.go:152

• The host-symlink path is cached in requestFileHandles, but this branch reads layer.base before consulting that cache. Every repeated compiler lookup through an explicit host symlink therefore performs another host ReadFile even though the newly read content is discarded when the cached handle is returned; check requestFileHandles before the host read so the snapshot cache actually avoids repeated I/O.

		if lookup.host {
			if content, ok := layer.base.ReadFile(lookup.path); ok {
				return s.requestFileHandle(originalFileName, originalPath, content)
			}

tsc/internal/project/snapshotfs.go:408

• This duplicate host-symlink branch bypasses the requestFileHandles cache until after layer.base.ReadFile has already run, so repeated auto/project lookups reread the host and then discard the result. Load the cached handle before calling the host filesystem, as in the lower-layer cache paths.

		if lookup.host {
			if content, ok := layer.base.ReadFile(lookup.path); ok {
				return s.requestFileHandle(originalFileName, originalPath, content)
			}

• Files reviewed: 26/26 changed files
• Comments generated: 2
• Review effort level: Lite (auto)

Suppressed comments (3)

tsc/internal/project/requestfilesystem.go:447

• For a full request filesystem, an absent path has lookup.fallback == false, but this branch still delegates to below.Realpath. That lets the host canonicalize a path that the full filesystem deliberately does not expose (for example, returning a host-cased path while FileExists/ReadFile are false). Return lookup.path when fallback is false and only consult below for layer fallthrough.
  tsc/internal/project/requestfilesystem.go:442
• On a case-insensitive host, lookup.info can resolve a request file or directory from a differently cased query (including after following a request symlink), but lookup.path retains the query/target spelling. Returning it here violates vfs.FS.Realpath's casing contract and can give different realpaths for the same request entry. Return the stored requestFile.fileName or requestDirectory.directoryName instead, and cover both direct and symlinked lookups.
  tsc/internal/project/requestfilesystem.go:496
• When directoryName is a request symlink (for example, /alias -> /target), this filters lower entries using /alias/name, but tombstones are stored under /target/name. GetAccessibleEntries("/alias") therefore still advertises a target path listed in RemovedPaths, even though GetFile("/alias/name") correctly returns nil. Filter using lookup.path so removals are applied after symlink resolution, and add a regression test for enumeration through an alias.

• Files reviewed: 26/26 changed files
• Comments generated: 0 new
• Review effort level: Lite (auto)