feat: Prevent implicit document creation via If-Match header

[karlpauls]

Problem

Currently, da-collab can implicitly create documents in da-admin through the first PUT request. This creates issues when documents are deleted or never existed - we end up recreating them
unintentionally.

Solution

This PR enforces explicit document existence checks at two levels:

1. Authorization Phase

Reject connections to non-existent documents immediately. Previously, 404 responses during the HEAD request were allowed (anticipating document creation). Now they fail fast at the edge.

2. Write Phase

Added If-Match: * header to all PUT requests. This tells da-admin "only write if document exists" - the server returns 412 if it doesn't.

3. Cleanup on Conflict

When we get 412 (document was deleted), we now:

• Clean the Durable Object storage cache via storage.deleteAll()
• Close all active WebSocket sessions
• Remove the ydoc from the global cache

4. Error Propagation

persistence.get now throws on 404 instead of returning null. This catches the edge case where a document is deleted between authorization and content fetch.

Error details are stored in the Yjs document's error map before connections close, so clients receive the error via the sync protocol.

Deployment

⚠️ da-admin must deploy If-Match support before this goes out

The header needs to be recognized and 412 returned when documents don't exist. See adobe/da-admin#187

[github-actions]

LCOV of commit daa15df during Install, lint, and test #260

Summary coverage rate:
  lines......: 99.1% (2094 of 2112 lines)
  functions..: 95.9% (71 of 74 functions)
  branches...: no data found

Files changed coverage rate:
                  |Lines       |Functions  |Branches    
  Filename        |Rate     Num|Rate    Num|Rate     Num
  ======================================================
  src/edge.js     | 2.7%    339| 0.0%     9|    -      0
  src/shareddoc.js| 3.0%    642| 0.0%    19|    -      0