feat: add dataplex-get-data-product tool

[theantagonist9509]

Important

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

• 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-get-data-product tool for the Dataplex
(Knowledge Catalog) source, allowing users to retrieve detailed metadata for
a specific Data Product.

Changes overview:

• Dataplex Source: Implemented GetDataProduct to fetch a single Data
  Product by location and ID. It maps SDK structures to a clean flat output,
  including display name, description, owner emails, and access groups.
• New Tool: Created the dataplex-get-data-product tool exposing locationId
  and dataProductId parameters.
• Tests:
  • Added unit tests for tool configuration parsing.
  • Added integration tests verifying successful retrieval of Data Products
    and proper 401 handling on unauthorized requests.
• Documentation: Created the reference documentation page for the tool,
  updated the Dataplex source guide, and added the tool details to the
  capabilities list.

[gemini-code-assist]

Code Review

This pull request adds support for Data Products in the Dataplex (Knowledge Catalog) integration by introducing two new tools: dataplex-list-data-products and dataplex-get-data-product. The changes include updating the Dataplex source to initialize and use the DataProductClient, implementing the new tool configurations, adding comprehensive integration tests, and updating the documentation. The code reviewer provided feedback recommending the consistent capitalization of initialisms like 'ID' (e.g., locationID, dataProductID) to align with Go style guidelines, correcting a typo in a parameter description, explicitly passing the cloud platform scope when finding default credentials in tests, and using t.Fatalf to fail fast during test setup if the service account email is missing.