fix(markup): link team mentions that use a different org name case (#39436) (#39588)

Backport #39436 by @breken-ai

The `org` render meta is lower-cased (`Repository.composeCommonMetas`
sets `metas["org"] = strings.ToLower(repo.OwnerName)`), but
`mentionProcessor` compares it with the org part of the mention exactly
as typed, while the team part is lower-cased before its lookup. So in a
repository of the org `MyOrg`, `@MyOrg/developers` stays plain text,
although `ResolveIssueMentionsByVisibility` lower-cases the name and
still notifies that team.

This compares the org part case-insensitively and adds a render test for
team mentions.

The code is the same on `release/v1.27`, so it could be backported.

### Verification
- `go test ./modules/markup/ -run TestRender_TeamMention -count=1`: the
`@Org1/Developers` case renders as plain text on current main and as the
team link with this change; `go test ./modules/markup/...` passes.
- `golangci-lint run ./modules/markup/` (v2.13.2): 0 issues.

### AI assistance
This contribution was prepared with AI assistance. The behavior, the
metas and mention-resolution code paths, the patch and the test were
reviewed manually.

> Built by breken, your AI support engineer - breken.ai - this one's on
us.

Co-authored-by: breken <support@brekfuz.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/html_test.go

@@ -605,3 +605,18 @@ func TestIssue18471(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, `<a href="`+markup.TestAppURL+`org/repo/compare/783b039...da951ce" class="compare"><code>783b039...da951ce</code></a>`, res.String())
 }
+
+func TestRender_TeamMention(t *testing.T) {
+	// the "org" meta is lower-cased, see Repository.composeCommonMetas
+	metas := map[string]string{"user": "Org1", "repo": "repo1", "org": "org1", "teams": ",developers,"}
+	test := func(input, expected string) {
+		rctx := markup.NewTestRenderContext(markup.TestAppURL, metas).WithRelativePath("a.md")
+		buffer, err := testRenderString(rctx, input)
+		assert.NoError(t, err)
+		assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(buffer))
+	}
+	test("@org1/developers", `<p><a href="/org/org1/teams/developers" rel="nofollow">@org1/developers</a></p>`)
+	test("@Org1/Developers", `<p><a href="/org/Org1/teams/Developers" rel="nofollow">@Org1/Developers</a></p>`)
+	test("@org2/developers", `<p>@org2/developers</p>`)
+	test("@org1/testers", `<p>@org1/testers</p>`)
+}