openapi3: fix validation of duplicated path templates

[reuvenharrison]

Extracted from #1187 per @fenollp review.

Bug

Paths.Validate keeps a dedup map to detect two paths that template to the same canonical form (/users/{id} vs /users/{name}). The lookup keys on the normalized path, but the insertion was keyed on the literal path:

if oldPath, ok := normalizedPaths[normalizedPath]; ok {   // lookup: normalized
    return fmt.Errorf("conflicting paths %q and %q", path, oldPath)
}
normalizedPaths[path] = path                              // insert: literal  ← bug

Each conflicting path got its own literal-keyed entry, so the lookup never matched. The check was unreachable.

Fix

One line: key the insertion on the normalized path, matching the lookup. Error text and types are unchanged; the path was just never being reached.

Impact

Across testdata/APIs-guru-openapi-directory, 14 real-world specs are now correctly reported as having conflicting paths (their new __validate goldens pin the corrected behaviour). No other golden fixture moves.