feat: add dataplex-list-data-assets tool

[theantagonist9509]

Important

Stacked Changeset: This PR depends on changes in previous PRs (#3337, #3499).

• Please merge previous PRs first before reviewing/merging this one.
• The file diff below will appear large as it includes its ancestors' commits. It will resolve itself once the preceding PRs are merged and this branch is synced.

This PR implements the dataplex-list-data-assets tool for the Dataplex
(Knowledge Catalog) source, enabling users to list and paginate all Data
Assets attached to a specific Data Product.

Changes overview:

• Dataplex Source: Implemented ListDataAssets which queries assets attached
  to a Data Product parent, supporting custom filtering, page size, and order
  parameters.
• New Tool: Created the dataplex-list-data-assets tool exposing locationId ,
  dataProductId , filter , pageSize , and orderBy parameters.
• Tests:
  • Added unit tests for tool configuration parsing.
  • Added integration tests asserting list results, page size constraints,
    and proper authorization validation.
• Documentation: Created the reference page for the tool, updated the
  Dataplex source guide, and added it to the main capabilities list.

[gemini-code-assist]

Code Review

This pull request introduces support for Dataplex Data Products and Data Assets in the Knowledge Catalog MCP server by adding three new tools: list_data_products, get_data_product, and list_data_assets. The changes include updating the Dataplex source to initialize and use the DataProductClient, implementing the tool configurations and invocation logic, adding integration tests, and updating the documentation. The review feedback suggests exporting the dataProductClient field as DataProductClient in the Source struct for consistency, updating its getter and initialization logic, and correcting a minor typo ('accross') in a parameter description.