test: add integration test for registry SchemaVersionTooNew error path

[breferrari]

Context

PR #237 added schema version validation to registry responses, but the SchemaVersionTooNew error path for parse_search_index and load_pack_metadata can only be tested with a mock HTTP server (the methods are private on GitHubRegistry). Unit tests cover deserialization and the version comparison logic, but no integration test exercises the full error path through the HTTP layer.

Proposal

Add a wiremock-based integration test that:

1. Serves an index.json with schema_version: 99
2. Verifies weave search returns the SchemaVersionTooNew error
3. Serves a packs/test.json with schema_version: 99
4. Verifies weave install test returns the SchemaVersionTooNew error

Acceptance criteria

• Integration test using wiremock for registry schema version rejection
• Test covers both index.json and packs/{name}.json paths