Allow known breaks in the version compatibility test

The version-compat job fails every pull request once master breaks
compatibility with the latest release on purpose. List those breaks in
scripts/version-compat-breaks.json keyed by the published version they
break against, each with a pr number and reason. Listed messages warn
instead of failing, connect skips the cluster test, entries for other
versions are ignored, and a listed message that did not change fails.

Add the two src changes from #1129 against 2.0.3 so master passes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/version_compat_breaks.js

@@ -0,0 +1,91 @@
+import assert from "node:assert/strict";
+
+import {
+	loadBreaks, matchBreaks, selectBreaks, validateBreaks,
+} from "../scripts/version-compat-breaks.js";
+
+
+describe("version compatibility known breaks", function() {
+	const breaks = {
+		"2.0.3": {
+			messages: {
+				"request ListedRequest": { pr: 1, reason: "Listed." },
+				"event UnchangedEvent": { pr: 2, reason: "Unchanged." },
+			},
+			connect: { pr: 3, reason: "Cannot connect." },
+		},
+		"2.0.2": {
+			messages: { "request OldRequest": { pr: 4, reason: "Old." } },
+		},
+	};
+
+	describe("validateBreaks()", function() {
+		it("should accept the committed file", async function() {
+			await loadBreaks();
+		});
+		it("should accept a valid file", function() {
+			validateBreaks(breaks);
+			validateBreaks({});
+		});
+		it("should require a pr number and reason", function() {
+			assert.throws(
+				() => validateBreaks({ "1.0.0": { messages: { "request A": { reason: "A." } } } }),
+				/must have a pr number/
+			);
+			assert.throws(
+				() => validateBreaks({ "1.0.0": { messages: { "request A": { pr: 1, reason: " " } } } }),
+				/must have a reason/
+			);
+			assert.throws(
+				() => validateBreaks({ "1.0.0": { connect: { pr: 1 } } }),
+				/connect must have a reason/
+			);
+		});
+		it("should reject unknown properties and bad message names", function() {
+			assert.throws(() => validateBreaks({ "1.0.0": { connected: false } }), /unknown properties connected/);
+			assert.throws(
+				() => validateBreaks({ "1.0.0": { messages: { "request A": { pr: 1, reason: "A.", x: 1 } } } }),
+				/unknown properties x/
+			);
+			assert.throws(
+				() => validateBreaks({ "1.0.0": { messages: { "A": { pr: 1, reason: "A." } } } }),
+				/must be named/
+			);
+		});
+	});
+
+	describe("selectBreaks()", function() {
+		it("should pick the entry for the published version", function() {
+			const selected = selectBreaks(breaks, "2.0.3");
+			assert.deepEqual([...selected.messages.keys()], ["request ListedRequest", "event UnchangedEvent"]);
+			assert.deepEqual(selected.connect, { pr: 3, reason: "Cannot connect." });
+			assert.deepEqual(selected.ignored, ["2.0.2"]);
+		});
+		it("should ignore every entry when the version is not listed", function() {
+			const selected = selectBreaks(breaks, "2.0.4");
+			assert.equal(selected.messages.size, 0);
+			assert.equal(selected.connect, undefined);
+			assert.deepEqual(selected.ignored, ["2.0.3", "2.0.2"]);
+		});
+	});
+
+	describe("matchBreaks()", function() {
+		const { messages } = selectBreaks(breaks, "2.0.3");
+		it("should split changed messages into known and unknown", function() {
+			const result = matchBreaks(messages, ["request ListedRequest", "event UnchangedEvent", "event OtherEvent"]);
+			assert.deepEqual(result, {
+				known: ["request ListedRequest", "event UnchangedEvent"],
+				unknown: ["event OtherEvent"],
+				stale: [],
+			});
+		});
+		it("should report listed messages that did not change as stale", function() {
+			const result = matchBreaks(messages, ["request ListedRequest"]);
+			assert.deepEqual(result.stale, ["event UnchangedEvent"]);
+		});
+		it("should not match messages listed for another version", function() {
+			const result = matchBreaks(selectBreaks(breaks, "2.0.3").messages, ["request OldRequest"]);
+			assert.deepEqual(result.unknown, ["request OldRequest"]);
+		});
+	});
+});