feat: Added prompt support for toolbox (#1798)

## Description

Added MCP prompt support in the toolbox server.

- No updates needed corresponding to
https://github.com/googleapis/genai-toolbox/pull/1828/files.

## 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 https://github.com/googleapis/genai-toolbox/issues/1040

---------

Co-authored-by: Yuan Teoh <45984206+Yuan325@users.noreply.github.com>
Co-authored-by: dishaprakash <57954147+dishaprakash@users.noreply.github.com>
Co-authored-by: Mend Renovate <bot@renovateapp.com>
Co-authored-by: Averi Kitsch <akitsch@google.com>
Co-authored-by: Anmol Shukla <shuklaanmol@google.com>
Co-authored-by: Harsh Jha <83023263+rapid-killer-9@users.noreply.github.com>
Co-authored-by: Wenxin Du <117315983+duwenxin99@users.noreply.github.com>
Co-authored-by: Kurtis Van Gent <31518063+kurtisvg@users.noreply.github.com>
Co-authored-by: Dr. Strangelove <drstrangelove@google.com>
Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
Co-authored-by: Dave Borowitz <dborowitz@google.com>

.github/workflows/tests.yaml

@@ -81,11 +81,12 @@ jobs:
         run: |
           source_dir="./internal/sources/*"
           tool_dir="./internal/tools/*"
+          prompt_dir="./internal/prompts/*"
           auth_dir="./internal/auth/*"
           int_test_dir="./tests/*"
-          included_packages=$(go list ./... | grep -v -e "$source_dir" -e "$tool_dir" -e "$auth_dir" -e "$int_test_dir")
+          included_packages=$(go list ./... | grep -v -e "$source_dir" -e "$tool_dir"  -e "$prompt_dir" -e "$auth_dir" -e "$int_test_dir")
           go test -race -cover -coverprofile=coverage.out -v $included_packages
-          go test -race -v ./internal/sources/... ./internal/tools/... ./internal/auth/...
+          go test -race -v ./internal/sources/... ./internal/tools/... ./internal/prompts/... ./internal/auth/...
 
       - name: Run tests without coverage
         if: ${{ runner.os != 'Linux' }}