chore(lsp): per-server stderr noise filter via suppressStderrPattern

Pyrefly narrates every request on stderr at INFO level, flooding the
console with error-level noise. registerLanguageServer now accepts an
optional suppressStderrPattern regex string (it crosses the node
connector, so not a RegExp); matching stderr lines are dropped from the
console log only. stderrTail still keeps the full text for crash
reports, an invalid pattern is ignored, and servers without a pattern
are untouched. PythonSupport opts in with ^\s*INFO\b.

Author: abose

src-node/lsp-client.js

@@ -236,10 +236,25 @@ exports.ping = async function ping() {
  * @param {Object} [params.workspaceConfiguration] - settings tree served to the server's
  *        workspace/configuration pulls (sections resolved by dotted path); pulls answer null
  *        without it
+ * @param {string} [params.suppressStderrPattern] - regex source; stderr lines matching it are
+ *        dropped from the live console log for this server (for servers that narrate every
+ *        request, e.g. pyrefly with "^\\s*INFO\\b"). The full stderr is still kept in
+ *        stderrTail for crash reports
  * @returns {Promise<Object>} Result with success status and server info
  */
 exports.startServer = async function startServer(params) {
-    const { serverId, command, args = ['--stdio'], rootUri, workspaceConfiguration } = params;
+    const { serverId, command, args = ['--stdio'], rootUri, workspaceConfiguration,
+        suppressStderrPattern } = params;
+
+    // Compiled once here; a broken pattern must not take the server down over log cosmetics.
+    let stderrDropRegex = null;
+    if (suppressStderrPattern) {
+        try {
+            stderrDropRegex = new RegExp(suppressStderrPattern);
+        } catch (err) {
+            console.error(`[lsp-client][${serverId}] invalid suppressStderrPattern ignored:`, err.message);
+        }
+    }
 
     if (!serverId || !command) {
         throw new Error('serverId and command are required');
@@ -291,7 +306,19 @@ exports.startServer = async function startServer(params) {
             if (serverState.stderrTail.length > 50) {
                 serverState.stderrTail.shift();
             }
-            console.error(`[lsp-client][${serverId} stderr]`, text.trimEnd());
+            // A server registered with suppressStderrPattern narrates routine traffic on stderr
+            // (e.g. pyrefly logs every request at INFO level), which drowns real problems in the
+            // console. Drop matching lines from the console log only - stderrTail above keeps the
+            // full text, so crash reports still carry everything.
+            let consoleText = text.trimEnd();
+            if (stderrDropRegex) {
+                consoleText = text.split('\n')
+                    .filter(line => line.trim() && !stderrDropRegex.test(line))
+                    .join('\n');
+            }
+            if (consoleText) {
+                console.error(`[lsp-client][${serverId} stderr]`, consoleText);
+            }
         });
 
         serverProcess.on('spawn', () => {

src/extensions/default/PythonSupport/main.js

@@ -105,7 +105,9 @@ define(function (require, exports, module) {
             // pyproject.toml stays authoritative over this.
             workspaceConfiguration: {
                 python: { pyrefly: { typeCheckingMode: "default" } }
-            }
+            },
+            // pyrefly narrates every request on stderr at INFO level - keep that out of the console
+            suppressStderrPattern: "^\\s*INFO\\b"
         });
         if (client) {
             registered = true;

src/languageTools/LSPClient.js

@@ -705,7 +705,8 @@ define(function (require, exports, module) {
             command: config.command,
             args: config.args || ["--stdio"],
             rootUri: rootUri,
-            workspaceConfiguration: config.workspaceConfiguration
+            workspaceConfiguration: config.workspaceConfiguration,
+            suppressStderrPattern: config.suppressStderrPattern
         });
 
         const initResult = await conn.execPeer("sendRequest", {
@@ -812,6 +813,10 @@ define(function (require, exports, module) {
      *        treats the default null answer as "all diagnostics off").
      * @param {function(Array):Array} [config.filterDiagnostics] - server-specific post-filter for
      *        published diagnostics
+     * @param {string} [config.suppressStderrPattern] - regex source (string, not RegExp - it
+     *        crosses the node connector); stderr lines matching it are dropped from the live
+     *        console log. Opt in for servers that narrate every request on stderr (pyrefly uses
+     *        "^\\s*INFO\\b"); the full stderr is still kept node-side for crash reports.
      * @return {Promise<LanguageClient|null>} the client, or null if it could not be started
      */
     async function registerLanguageServer(config) {