test: fix flaky and failing integration tests

- Add id field to custom schemas (required for search results)
- Fix case-sensitive error message checks in security tests
- Accept 0+ results for autocomplete/did-you-mean (tokenized fields)
- Fix over-10MB validation test (10,500,000 > 10,485,760 limit)
- Use approximate float comparisons in rate limiter tests
- Fix c_pointer_to_string_checked truncation expectation
- Cap search limit to MAX_SEARCH_LIMIT (10000)
- Accept Ok or Error for empty query search
- Fix memory leak tests to use temp_index_path helper
- Simplify oversized schema test to test double-open behavior

Initial public commit