[api] Fix two filename case sensitivity issues

[Andrew Branch (andrewbranch)]

• Preserve original Windows drive letter case across API boundary 4da8232

• On case-insensitive file systems, creating a snapshot VFS would infer directories case-sensitively on the client, while the server rejects duplicates case-insensitively, such that if you passed createFileSystem({ files: [["C:/Proj/main.ts"], [""], ["C:/proj/foo.ts"], [""]] }), this would be inferred as two distinct directories C:/Proj and C:/proj, but then be rejected by the server as duplicates.

  d02db3a fixed this on the server side by merging duplicates instead of erroring, but I didn't like how that blurred the difference between automatically inferred directories from case-inconsistent inputs and various real user-authored mistakes. I couldn't see a strong reason for the client to infer directories up front when the server already does, so the next commit removed the client-side inference in favor of just letting the server do it with its case-sensitivity knowledge.

[Copilot]

Copilot review overview

🟢 Approval recommended

The casing and directory-inference changes are consistent and covered across server and client APIs.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes Windows drive-letter casing across the API boundary and moves directory inference to the case-aware server.

Changes:

• Preserves drive-letter casing when decoding file URIs.
• Derives omitted directory listings server-side deterministically.
• Updates sync, async, and Go regression tests.

File	Description
tsc/​internal/​lsp/​lsproto/​lsp.go	Preserves URI drive-letter case.
tsc/​internal/​ls/​lsconv/​converters_test.go	Updates URI conversion expectations.
tsc/​internal/​api/​session_createprogram_test.go	Tests snapshot drive-letter preservation.
tsc/​internal/​api/​requestfilesystem/​requestfilesystem.go	Deterministically derives directory structure.
tsc/​internal/​api/​requestfilesystem/​requestfilesystem_test.go	Tests host-aware casing behavior.
packages/​typescript/​test/​sync/​api.test.ts	Updates synchronous API expectations.
packages/​typescript/​test/​async/​api.test.ts	Updates asynchronous API expectations.
packages/​typescript/​src/​api/​proto.generated.ts	Documents server-side derivation.
packages/​typescript/​src/​api/​path.ts	Preserves casing during URI decoding.
packages/​typescript/​src/​api/​fs.ts	Removes client-side directory inference.

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[Andrew Branch (andrewbranch)]

Your comment made me realize that there was still a path that went from file name to URI and back to file name. If either one of those conversions is intended to be lossy/case-normalizing, any API requests that can end up creating filenames on new source files shouldn't go through that. Fixed in the latest commit.