fix(toolcall): eliminate strings.ToLower panics from Unicode case folding

Replace all strings.ToLower usage with ASCII case-insensitive matching
(hasASCIIPrefixFoldAt, indexASCIIFold, hasDSMLPrefix) to prevent slice
bounds errors when Unicode characters change byte length after case
folding (e.g., Turkish İ U+0130 → i + combining dot: 2 bytes → 3 bytes).

Root cause: code created a strings.ToLower(text) copy, found byte
positions in that copy, then used those positions to slice the
original text — byte offsets that were valid in the lowercased copy
became out-of-bounds in the original when case folding changed byte
lengths.

Files changed:
- toolcalls_scan.go: remove 5 lower usages, add hasDSMLPrefix
- toolcalls_parse_markup.go: remove 3 lower usages, add indexASCIIFold
- toolcalls_markup.go: SanitizeLooseCDATA lower removal
- toolcalls_parse.go: updateCDATAStateForStrip lower removal
- tool_prompt.go: align DSML pipe characters with tool call spec
- tool_prompt_test.go: fix pre-existing test character mismatch

internal/promptcompat/prompt_build_test.go

@@ -74,7 +74,7 @@ func TestBuildOpenAIFinalPrompt_VercelPreparePathKeepsFinalAnswerInstruction(t *
 	}
 
 	finalPrompt, _ := buildOpenAIFinalPrompt(messages, tools, "", false)
-	if !strings.Contains(finalPrompt, "Remember: The ONLY valid way to use tools is the <|DSML|tool_calls>...</|DSML|tool_calls> block at the end of your response.") {
+	if !strings.Contains(finalPrompt, "Remember: The ONLY valid way to use tools is the <｜DSML｜tool_calls>...<｜/DSML｜tool_calls> block at the end of your response.") {
 		t.Fatalf("vercel prepare finalPrompt missing final tool-call anchor instruction: %q", finalPrompt)
 	}
 	if !strings.Contains(finalPrompt, "TOOL CALL FORMAT") {