Add path resolution and error handling for prompt file path parameters

- Add resolvePromptFilePath() utility that resolves relative paths against
  workspace root and returns user-friendly warnings for invalid paths
- Update all 9 prompt handlers with file path parameters to resolve paths
  and embed warnings in the response instead of throwing protocol errors
- Add integration test in mcp-test-suite.js for explain_codeql_query with
  invalid path
- Add file-based integration test fixtures in primitives/prompts/
- Add 10 unit tests for resolvePromptFilePath and handler path resolution

Co-authored-by: data-douser <70299490+data-douser@users.noreply.github.com>

Author: Copilot

client/integration-tests/primitives/prompts/explain_codeql_query/invalid_query_path/README.md

@@ -0,0 +1,21 @@
+# Integration Test: explain_codeql_query / invalid_query_path
+
+## Purpose
+
+Verify that the `explain_codeql_query` prompt returns a helpful error message
+when the user provides a `queryPath` that does not exist on disk.
+
+## Expected Behavior
+
+- The prompt handler resolves the relative path against the workspace root.
+- When the resolved path does not exist, the prompt returns a warning in the
+  response messages rather than throwing a raw MCP protocol error.
+- The warning message should mention the file was not found so the user can
+  correct the path.
+
+## Parameters
+
+| Parameter   | Value                          | Notes              |
+| ----------- | ------------------------------ | ------------------ |
+| `queryPath` | `nonexistent/path/to/query.ql` | Does **not** exist |
+| `language`  | `javascript`                   | Valid language     |

client/integration-tests/primitives/prompts/explain_codeql_query/invalid_query_path/after/monitoring-state.json

@@ -0,0 +1,3 @@
+{
+  "sessions": []
+}

client/integration-tests/primitives/prompts/explain_codeql_query/invalid_query_path/before/monitoring-state.json

@@ -0,0 +1,3 @@
+{
+  "sessions": []
+}

client/src/lib/mcp-test-suite.js

@@ -22,6 +22,7 @@ export class MCPTestSuite {
     await this.testReadResource();
     await this.testListPrompts();
     await this.testGetPrompt();
+    await this.testGetPromptWithInvalidPath();
   }
 
   /**
@@ -132,6 +133,58 @@ export class MCPTestSuite {
     }
   }
 
+  /**
+   * Test that prompts handle invalid file paths gracefully.
+   *
+   * The explain_codeql_query prompt requires a `queryPath` parameter.
+   * When given a nonexistent path it should return a user-friendly error
+   * message inside the prompt response rather than throwing a raw MCP
+   * protocol error.
+   */
+  async testGetPromptWithInvalidPath() {
+    try {
+      this.logger.log("Testing prompt error handling with invalid file path...");
+
+      const result = await this.client.getPrompt({
+        name: "explain_codeql_query",
+        arguments: {
+          queryPath: "nonexistent/path/to/query.ql",
+          language: "javascript"
+        }
+      });
+
+      // The prompt should return messages (not throw) even for an invalid path
+      const hasMessages = result.messages && result.messages.length > 0;
+      if (!hasMessages) {
+        this.logger.logTest(
+          "Get Prompt with Invalid Path (explain_codeql_query)",
+          false,
+          "Expected messages in response"
+        );
+        return false;
+      }
+
+      // The response should contain a human-readable error about the invalid path
+      const text = result.messages[0]?.content?.text || "";
+      const mentionsPathError =
+        text.includes("does not exist") ||
+        text.includes("not found") ||
+        text.includes("could not be found") ||
+        text.includes("File not found") ||
+        text.includes("⚠");
+
+      this.logger.log(`Prompt response mentions path error: ${mentionsPathError}`);
+      this.logger.logTest("Get Prompt with Invalid Path (explain_codeql_query)", mentionsPathError);
+
+      return mentionsPathError;
+    } catch (error) {
+      // If the server throws instead of returning a message this test fails,
+      // which is the exact behaviour we want to fix.
+      this.logger.logTest("Get Prompt with Invalid Path (explain_codeql_query)", false, error);
+      return false;
+    }
+  }
+
   /**
    * Test listing prompts
    */