fix(adapters): preserve gateway JSON parse context

[xy200303]

Summary

• Adds GatewayMemoryClientParseError for successful Gateway responses that cannot be parsed as JSON.
• Preserves the request path, HTTP status, and raw response body so platform adapter integrations can diagnose malformed Gateway responses.
• Exports the new error type from both adapter barrels and the package entry point.

Relationship to upstream work

• This is a small follow-up patch for feat(adapters): add gateway client adapter kit TencentCloud/TencentDB-Agent-Memory#316.
• It does not add a new adapter implementation or change the feat(adapters): add gateway client adapter kit TencentCloud/TencentDB-Agent-Memory#316 Gateway client API shape beyond exposing a more specific parse error.
• Related to Cross-Platform Adapters for the Memory Plugin TencentCloud/TencentDB-Agent-Memory#235.

Verification

• npm install --no-package-lock --ignore-scripts
• npm test -- src/adapters/gateway-client/gateway-client.test.ts
• git diff --check upstream/pr/316...HEAD