fix(artifacts): add a worked edit example to reduce empty update blocks

An N=70 eval (GLM-5.2) found ~11% of edits emit a bare <artifact type="update"></artifact> with no pairs. The prompt only showed the placeholder schema. Add a concrete old_str/new_str example and an explicit rule that an update block must contain at least one pair (else re-emit the full artifact with the same identifier).

Author: gary149

src/lib/server/textGeneration/artifacts.ts

@@ -36,7 +36,15 @@ Editing an artifact you created earlier in the conversation:
 <new_str>replacement text</new_str>
 </artifact>
 
+For example, to recolor a button in an existing "signup-form" artifact, emit exactly:
+
+<artifact identifier="signup-form" type="update">
+<old_str>background: #16a34a;</old_str>
+<new_str>background: #2563eb;</new_str>
+</artifact>
+
 - Each old_str must match the latest version EXACTLY (including whitespace/indentation) and must be unique within it. Copy it verbatim from the latest version; do not retype, reformat, or re-indent it.
+- Every update block must contain at least one complete old_str/new_str pair — never emit an empty type="update" block. If you can't produce exact old_str text, re-emit the full artifact instead (same identifier).
 - Emit at most ONE update block per reply, with all the pairs (up to 4) inside that single block — never one block per pair.
 - For larger changes, re-emit the full artifact with the SAME identifier (this creates a new version).
 - Keep the identifier BYTE-IDENTICAL across every version, even when the title or content changes (renaming a green button to blue keeps the same identifier). Use a new identifier only for a genuinely different artifact.