fix: 修复 4 个测试失败（路径规范化、SDK 签名变更、空消息防护）

- projectContext.test.ts: 使用 realpathSync 处理 macOS /var→/private/var 符号链接
- bedrockClient.test.ts: 适配 Bedrock SDK v0.80 Bearer 认证（原 AWS4-HMAC-SHA256）
- bridge.ts: forwardSessionUpdates 添加 null guard 防止空消息导致 TypeError

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/services/skillLearning/__tests__/projectContext.test.ts

@@ -1,5 +1,5 @@
 import { afterAll, beforeEach, describe, expect, test } from 'bun:test'
-import { existsSync, mkdirSync, mkdtempSync, readFileSync, rmSync } from 'fs'
+import { existsSync, mkdirSync, mkdtempSync, readFileSync, realpathSync, rmSync } from 'fs'
 import { tmpdir } from 'os'
 import { join } from 'path'
 import { execFileSync } from 'child_process'
@@ -56,7 +56,7 @@ describe('resolveProjectContext', () => {
 
     expect(context.source).toBe('claude_project_dir')
     expect(context.scope).toBe('project')
-    expect(context.projectRoot).toBe(projectDir)
+    expect(context.projectRoot).toBe(realpathSync(projectDir))
     expect(context.projectName).toBe(lastPathSegment(projectDir))
     expect(context.storageDir).toContain(context.projectId)
 
@@ -99,7 +99,7 @@ describe('resolveProjectContext', () => {
 
     expect(context.source).toBe('git_root')
     expect(context.scope).toBe('project')
-    expect(context.projectRoot).toBe(repo)
+    expect(context.projectRoot).toBe(realpathSync(repo))
     expect(context.projectName).toBe(lastPathSegment(repo))
   })