Fix JsonSafeParse violations in MCP ApiService

Doby Baxterrequested to merge
gitlab-community/gitlab-org/gitlab:mcp-json-safe-parse into master

What does this MR do?

Replaces Gitlab::Json.parse with Gitlab::Json.safe_parse in the MCP API service to address RuboCop JsonSafeParse violations.

Why is this change needed?

Gitlab::Json.safe_parse provides built-in protections against excessively large or deeply nested JSON payloads while remaining compatible with existing GitLab API responses.

Verification

The JSON parsed in this service originates from GitLab API responses and is expected to remain within the size and depth limits enforced by safe_parse.

Related issue

Closes #586025 (closed)

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