Replace JSON validator in workflow

[jasonjoh]

The old validator orrosenblatt/validate-json-action hasn't been updated since 2019 and was using deprecated tech. For example, https://github.com/microsoftgraph/microsoft-graph-devx-content/actions/runs/17332688629:

The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Updating to dsanders11/json-schema-validate-action gives the following advantages:

• It detected a syntax error in the schema itself that the old validator didn't catch: strict mode: "items" is 1-tuple, but minItems or maxItems/additionalItems are not specified or different at path "#/properties/SampleQueries/items/properties/headers"
• It fails validation when properties are repeated, which fixes Investigate schema validation to prevent multiple entries of the same property.  #408
• It is (at least for now) being actively maintained

[Copilot]

Pull Request Overview

This PR updates the JSON validation workflow by replacing the deprecated orrosenblatt/validate-json-action with dsanders11/json-schema-validate-action and fixes schema syntax issues that the new validator detected.

• Replaced deprecated JSON validation action with actively maintained alternative
• Fixed JSON schema syntax by converting tuple-based item definitions to object-based definitions
• Updated GitHub Actions checkout versions and added workflow dispatch trigger

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/workflows/validate.yml	Replaced deprecated JSON validator action and updated checkout versions
tests/samples.schema.json	Fixed schema syntax by converting tuple-based items to object-based and added schema version

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