Compose: Refine the check_overflow_text function

Fixes: zulip#28706

This commit optimization in compose_validate.js manages cases where text.length exceeds max_length, displaying exceeded messages and negative exceeded characters, and tooltip message is updated to 'Maximum message length: 10000 characters.’This simplification enhances clarity by displaying only the remaining characters.

web/tests/compose_validate.test.js

@@ -588,7 +588,7 @@ test_ui("test_check_overflow_text", ({mock_template}) => {
     $textarea.val("a".repeat(10000 + 1));
     compose_validate.check_overflow_text();
     assert.ok($indicator.hasClass("over_limit"));
-    assert.equal(limit_indicator_html, "-100\n");
+    assert.equal(limit_indicator_html, "-1\n\n");
     assert.ok($textarea.hasClass("over_limit"));
     assert.ok(banner_rendered);
     assert.ok($(".message-send-controls").hasClass("disabled-message-send-controls"));
@@ -598,7 +598,7 @@ test_ui("test_check_overflow_text", ({mock_template}) => {
     $textarea.val("a".repeat(9000 + 1));
     compose_validate.check_overflow_text();
     assert.ok(!$indicator.hasClass("over_limit"));
-    assert.equal(limit_indicator_html, "9001\n");
+    assert.equal(limit_indicator_html, "999\n\n");
     assert.ok(!$textarea.hasClass("over_limit"));
     assert.ok(!$(".message-send-controls").hasClass("disabled-message-send-controls"));
     assert.ok(!banner_rendered);