fix: parse approve/unapprove response with correct schema

[millerchou]

Problem

approve_merge_request and unapprove_merge_request fail with:

MCP error -32603: Invalid arguments: approved_by.0.username: Required,
approved_by.0.name: Required, approved_by.0.state: Required,
approved_by.0.web_url: Required

Root Cause

The GitLab POST /approve and POST /unapprove endpoints return the approvals format where approved_by contains nested objects:

{ "approved_by": [{ "user": { "username": "X", "name": "Y", ... } }] }

But approveMergeRequest() and unapproveMergeRequest() parse the response with GitLabMergeRequestApprovalStateSchema, which expects flat user objects:

{ "approved_by": [{ "username": "X", "name": "Y", ... }] }

This worked before because the schema didn't include approved_by. After bfc49de added approved_by to the schema, Zod validation fails on the nested structure.

Fix

Extract a shared parseApprovalsResponse() helper that:

1. Parses with GitLabMergeRequestApprovalsResponseSchema (correct nested format)
2. Flattens { user: {...} } to {...}
3. Returns the unified GitLabMergeRequestApprovalState format

This is the same conversion that getMergeRequestApprovalsFallback() already does correctly — the fix reuses it for approveMergeRequest(), unapproveMergeRequest(), and the fallback itself.

Changes

• index.ts: 3 call sites updated, 1 new helper function, net +13/-5 lines