[Repo Assist] fix: encode non-ASCII path chars as UTF-8 percent sequences in FilePathToUri

[github-actions]

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Closes #840

Root Cause

FilePathToUri in Utils.fs was encoding non-ASCII characters in the U+0080–U+00FF range (accented letters, e.g. ó in código) as a single-byte %XX value derived from the Unicode code point. For ó (U+00F3) this produced %F3.

However, the LSP URI specification (and RFC 3986) require percent-encoded bytes to use UTF-8. In UTF-8, ó is two bytes — 0xC3 0xB3 — so the correct encoding is %C3%B3.

When a client (or .NET's own Uri.LocalPath) receives %F3, it interprets it as a standalone byte that starts an incomplete 4-byte UTF-8 sequence. The path is left as c%F3digo rather than código, breaking go-to-definition, diagnostics, and file-open for any project living in a directory whose name contains such characters.

Fix

Changed the else branch in FilePathToUri to obtain the UTF-8 byte representation of each character via Text.Encoding.UTF8.GetBytes and emit one %XX segment per byte:

// before
uri.Append('%')
(int c).TryFormat(buffer.Span, &out, "X2") |> ignore   // ó -> %F3 ❌

// after
let bytes = Text.Encoding.UTF8.GetBytes(string c)
for b in bytes do
    uri.Append('%')
    (int b).TryFormat(buffer.Span, &out, "X2") |> ignore  // ó -> %C3%B3 ✓

ASCII characters (space → %20, # → %23, etc.) are unaffected because their single-byte UTF-8 encoding equals their code point. Characters above U+00FF (ideographs, etc.) already passed through unencoded and are unchanged.

Trade-offs

• Minimal allocation per character encode (GetBytes(string c) for non-ASCII chars). These paths are encoded rarely (once per opened document), so the cost is negligible.
• Characters above U+00FF still pass through as bare Unicode (IRI-style). This matches the previous behaviour and avoids breaking any existing round-trips.

Test Status

• Build: ✅ dotnet build src/FsAutoComplete.Core -f net8.0 — 0 errors, 0 warnings
• URI unit tests: ✅ All 42 URI tests pass (--filter "Uri tests") including the new test:
  FilePathToUri encodes accented chars as UTF-8 percent sequences — verifies ó → %C3%B3

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[github-actions]

✅ Pull request created: #1455