## Description
This PR adds two read-only, PostgreSQL tool kinds to MCPToolbox that
standardize discovery of extensions:
- `postgres-list-installed-extensions` – lists extensions installed in
the current database with: `name`, `version`, `schema`, `owner`
`description`.
- `postgres-list-available-extensions` – lists extensions available to
install on the cluster with: `name`, `default_version`, `description`.
Both are safe by default, and return well-typed JSON suitable for UIs,
agents, and automation.
**Test Output**
- `list_installed_extensions`
<img width="1414" height="781" alt="image"
src="https://github.com/user-attachments/assets/aab102a7-90ab-4516-9bc2-bcbfdab02bb1"
/>
- `list_available_extensions`
<img width="1421" height="774" alt="image"
src="https://github.com/user-attachments/assets/a4f80f22-bb2e-4350-acb2-12eb020d88cb"
/>
---
> Should include a concise description of the changes (bug or feature),
it's
> impact, along with a summary of the solution
## PR Checklist
---
> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:
- [x] Make sure you reviewed
[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [x] Make sure to open an issue as a
[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
before writing your code! That way we can discuss the change, evaluate
designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change
🛠️ Fixes #<issue_number_goes_here>
