fix: validate separator newline in apply_diff to catch malformed ------- markers

When the LLM omits the newline after the ------- separator (e.g. "-------import"),
the parser silently absorbs the separator into the search content, producing a
confusing "No sufficiently similar match found" error. This causes smaller models
to loop without recovering.

Add detection in validateMarkerSequencing for lines starting with "-------" that
have non-whitespace content appended, returning a clear actionable error message
that tells the model to place the separator on its own line.

Closes #12210

src/core/diff/strategies/__tests__/multi-search-replace.spec.ts

@@ -131,6 +131,43 @@ describe("MultiSearchReplaceDiffStrategy", () => {
 			expect(result.error).toContain("Expected '>>>>>>> REPLACE' was not found")
 		})
 
+		it("detects malformed separator with content on the same line (no newline after -------)", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				":start_line:7\n" +
+				"-------import { useTranslate } from '../../i18n/I18nContext';\n" +
+				"=======\n" +
+				"new content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("separator line '-------'")
+			expect(result.error).toContain("must be on its own line")
+			expect(result.error).toContain("import { useTranslate }")
+		})
+
+		it("detects malformed separator without start_line", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" + "-------some content here\n" + "=======\n" + "new content\n" + ">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(false)
+			expect(result.error).toContain("separator line '-------'")
+			expect(result.error).toContain("some content here")
+		})
+
+		it("allows properly formatted separator on its own line", () => {
+			const diff =
+				"<<<<<<< SEARCH\n" +
+				":start_line:7\n" +
+				"-------\n" +
+				"import { useTranslate } from '../../i18n/I18nContext';\n" +
+				"=======\n" +
+				"new content\n" +
+				">>>>>>> REPLACE"
+			const result = strategy["validateMarkerSequencing"](diff)
+			expect(result.success).toBe(true)
+		})
+
 		describe("exact matching", () => {
 			let strategy: MultiSearchReplaceDiffStrategy
 

src/core/diff/strategies/multi-search-replace.ts

@@ -216,6 +216,27 @@ export class MultiSearchReplaceDiffStrategy implements DiffStrategy {
 					if (marker.startsWith(SEARCH_PREFIX)) return reportMergeConflictError(marker, SEARCH)
 					if (marker === REPLACE) return reportInvalidDiffError(REPLACE, SEP)
 					if (marker.startsWith(REPLACE_PREFIX)) return reportMergeConflictError(marker, SEARCH)
+					// Detect malformed separator: `-------` with content on the same line (missing newline)
+					if (/^-------\S/.test(marker)) {
+						const trailingContent = marker.slice(7)
+						return {
+							success: false,
+							error:
+								`ERROR: The separator line '-------' at line ${state.line} must be on its own line followed by a newline. ` +
+								`Found content '${trailingContent.length > 40 ? trailingContent.slice(0, 40) + "..." : trailingContent}' on the same line.\n` +
+								"\n" +
+								"CORRECT FORMAT:\n\n" +
+								"<<<<<<< SEARCH\n" +
+								":start_line:5\n" +
+								"-------\n" +
+								"search content here\n" +
+								"=======\n" +
+								"replacement content\n" +
+								">>>>>>> REPLACE\n" +
+								"\n" +
+								"The '-------' separator must be on its own line, with the search content starting on the next line.",
+						}
+					}
 					if (marker === SEP) state.current = State.AFTER_SEPARATOR
 					break