feat(auth): harden OAuth state generation and callback validation (closes #1562)

generateOAuthState() now throws when crypto.getRandomValues is unavailable
instead of silently degrading to a non-cryptographic Math.random() fallback —
OAuth state is a CSRF token and must be unpredictable.

The web /oauth/callback handler now rejects a returned `state` param that does
not parse to the expected 64-char-hex authId shape (a forgery indicator),
surfacing a clear error toast instead of proceeding with an undefined session.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3fTN8H3R8YV4yUGvZjYnX

clients/web/src/test/core/auth/utils.test.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect } from "vitest";
+import { describe, it, expect, afterEach } from "vitest";
 import {
   parseHttpUrl,
   parseOAuthCallbackParams,
@@ -51,6 +51,17 @@ describe("parseOAuthCallbackParams", () => {
 });
 
 describe("generateOAuthState", () => {
+  const originalCrypto = globalThis.crypto;
+
+  afterEach(() => {
+    // Restore the real WebCrypto after tests that stub/remove it.
+    Object.defineProperty(globalThis, "crypto", {
+      configurable: true,
+      writable: true,
+      value: originalCrypto,
+    });
+  });
+
   it("should generate 64-char hex state", () => {
     const state = generateOAuthState();
     expect(state).toMatch(/^[a-f0-9]{64}$/i);
@@ -61,6 +72,31 @@ describe("generateOAuthState", () => {
     const s2 = generateOAuthState();
     expect(s1).not.toBe(s2);
   });
+
+  it("throws when the crypto global is entirely absent (no silent Math.random fallback)", () => {
+    // Simulate an exotic runtime with no WebCrypto at all. Rather than minting
+    // a guessable CSRF token, generateOAuthState must fail loudly.
+    Object.defineProperty(globalThis, "crypto", {
+      configurable: true,
+      writable: true,
+      value: undefined,
+    });
+    expect(() => generateOAuthState()).toThrow(
+      /crypto\.getRandomValues is not available/,
+    );
+  });
+
+  it("throws when crypto.getRandomValues is missing", () => {
+    // crypto exists but lacks getRandomValues — still refuse to degrade.
+    Object.defineProperty(globalThis, "crypto", {
+      configurable: true,
+      writable: true,
+      value: {},
+    });
+    expect(() => generateOAuthState()).toThrow(
+      /crypto\.getRandomValues is not available/,
+    );
+  });
 });
 
 describe("parseOAuthState", () => {