Chat Agent bugfix to treat workspace file references and attachments separately

[GeorgeNgMsft]

Fix chat agent crash on highlighted editor files

Problem

When a user highlights lines in an editor (no upload) and asks a question, the
dispatcher's [Editor context] carries the workspace-relative file path, and the
LLM puts it into chat.generateResponse.relatedFiles. The chat agent assumed
every related file was an uploaded attachment staged under the session's
user_files/, joined the name there, and sessionStorage.read threw ENOENT —
failing the entire response. It also tried to embed every related file as an
image and only handled Windows path separators.

Fix

• New shared helper rehydrateImageAttachments in typechat-utils (used by
  both chat and dispatcher/search, replacing two near-identical copies). It
  gates on image MIME before any storage read, so a highlighted .yml is
  skipped with zero I/O — no read, no crash. Missing image uploads are caught and
  skipped. Also fixes a latent doubled-image/ data-URL bug and makes the
  user_files lookup portable via path.join (the old \..\user_files\ only
  resolved on Windows).
• Correct entity typing (relatedFileToEntity): uploaded images →
  ["file","image","data"]; other references → ["file"] (was hardcoded to
  image for everything).
• Schema nudge: tightened the relatedFiles comment so the model only lists
  uploaded attachment names, not editor/workspace files.