stream_data: Fix parsing of slugs for inaccessible channels.

Previously, if we had syntax in a URL slug that looked like a channel
ID, but we couldn't find the channel (say, beacuse it's a link to a
channel we're not subscribed to), parse_narrow would fail to parse it,
resulting in incorrect error handling.

This could break rendering of topic links that we want to process via
rendered_markdown.ts. I've confirmed that the web app's logic for
processing message_view.show does not require its caller to check the
channel ID is accessible.

The updated logic matches what we do in the mobile apps.

web/src/stream_data.ts

@@ -294,6 +294,12 @@ export function slug_to_stream_id(slug: string): number | undefined {
         return stream.stream_id;
     }
 
+    // Neither format found a channel, so it's inaccessible or doesn't
+    // exist. But at least we have a stream ID; give that to the caller.
+    if (newFormatStreamId) {
+        return newFormatStreamId;
+    }
+
     // Unexpected shape, or the old shape and we don't know of a stream with
     // the given name.
     return undefined;

web/tests/hash_util.test.cjs

@@ -185,10 +185,10 @@ run_test("test_parse_narrow", () => {
 
     assert.equal(hash_util.parse_narrow(["narrow", "BOGUS"]), undefined);
 
-    // For nonexistent streams, we get an empty string. We don't use this
-    // anywhere, and just show "Invalid stream" in the navbar.
+    // For unknown channel IDs, we still parse it; it could be a valid channel we do
+    // not have access to. We'll end up showing "Invalid stream" in the navbar.
     assert.deepEqual(hash_util.parse_narrow(["narrow", "stream", "42-bogus"]), [
-        {negated: false, operator: "stream", operand: ""},
+        {negated: false, operator: "stream", operand: "42"},
     ]);
 
     // Empty string as a topic name is valid.

web/tests/stream_data.test.cjs

@@ -217,18 +217,17 @@ test("basics", () => {
     assert.equal(stream_data.slug_to_stream_id("social"), 2);
     assert.equal(hash_util.decode_operand("channel", "social"), "2");
 
-    // These aren't prepended with valid ids nor valid channel names. We
-    // don't get any stream id from the slug, and the decoded operand (the
-    // only caller of `slug_to_stream_id`) returns an empty string (which we
-    // don't display anywhere, since the channel is invalid).
-    assert.equal(stream_data.slug_to_stream_id("999-social"), undefined);
-    assert.equal(hash_util.decode_operand("channel", "999-social"), "");
+    // These aren't prepended with valid ids nor valid channel names. Return
+    // the channel ID, since almost all URLs are the modern format and the
+    // most likely explanation is an inaccessible channel.
+    assert.equal(stream_data.slug_to_stream_id("999-social"), 999);
+    assert.equal(hash_util.decode_operand("channel", "999-social"), "999");
 
-    assert.equal(stream_data.slug_to_stream_id("99-whatever"), undefined);
-    assert.equal(hash_util.decode_operand("channel", "99-whatever"), "");
+    assert.equal(stream_data.slug_to_stream_id("99-whatever"), 99);
+    assert.equal(hash_util.decode_operand("channel", "99-whatever"), "99");
 
-    assert.equal(stream_data.slug_to_stream_id("25-or-6-to-4"), undefined);
-    assert.equal(hash_util.decode_operand("channel", "25-or-6-to-4"), "");
+    assert.equal(stream_data.slug_to_stream_id("25-or-6-to-4"), 25);
+    assert.equal(hash_util.decode_operand("channel", "25-or-6-to-4"), "25");
 
     // If this is the name of a stream, its id is returned.
     const stream_starting_with_25 = {
@@ -242,8 +241,8 @@ test("basics", () => {
     assert.equal(stream_data.slug_to_stream_id("2something"), undefined);
     assert.equal(hash_util.decode_operand("channel", "2something"), "");
 
-    assert.equal(stream_data.slug_to_stream_id("99"), undefined);
-    assert.equal(hash_util.decode_operand("channel", "99"), "");
+    assert.equal(stream_data.slug_to_stream_id("99"), 99);
+    assert.equal(hash_util.decode_operand("channel", "99"), "99");
     // If this is the name of a stream, its id is returned.
     const stream_99 = {
         name: "99",