feat: workflow to run tests

[harshiv-26]

Note

Medium Risk
Adds a comment-triggered CI path that can start external TrueFoundry jobs and uses repo secrets; while author checks and input sanitization reduce risk, misconfiguration could still lead to unintended job triggers or cost.

Overview
Adds a new GitHub Actions workflow (test-changed-models.yml) that listens for /test-models PR comments from trusted users, checks out the PR head separately, and triggers TrueFoundry gateway-test-job-v2 runs.

Introduces trigger_changed_model_tests.py to git diff the PR’s tip commit for changed providers/**/*.yaml, group changes by provider/model with basic path validation, and invoke tfy trigger job once per provider while reporting the number of triggered runs back to the workflow. Also updates .gitignore to ignore /scripts/ at the repo root.

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