Port type checking from mypy to ty

[pamelafox]

Purpose

Port Python type checking from mypy to ty.

This updates the development dependency, CI type-check command, repo configuration, and related documentation/checklists. It also includes small type-checker compatibility updates for OpenAI Agents stream event narrowing and an eval-only ty suppression.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[ ] Yes
[x] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[x] Other... Please describe: Development tooling change

Code quality checklist

See CONTRIBUTING.md for more details.

• The current tests all pass (python -m pytest).
• I added tests that prove my fix is effective or that my feature works
• I ran python -m pytest --cov to verify 100% coverage of added lines
• I ran python -m ty check to check for type errors
• I either used the pre-commit hooks or ran ruff manually on my code.

Validation run:

• ruff check .
• /usr/local/bin/python -m ty check . --python-version 3.12 --extra-search-path /home/vscode/.local/lib/python3.12/site-packages

[Copilot]

Pull request overview

Replaces the mypy type checker with ty across the project — updating dev dependencies, CI workflow, configuration, ignore files, IDE settings, contributor docs, and a couple of small in-code adjustments needed for ty compatibility.

Changes:

• Swap mypy → ty in requirements-dev.txt, CI workflow, pyproject.toml config, and docs/templates.
• Replace event.type == "raw_response_event" checks with isinstance(event, RawResponsesStreamEvent) in both RAG modules so ty can narrow event.data.
• Switch a # type: ignore to a # ty: ignore suppression in evals/generate_ground_truth.py.

Show a summary per file

File	Description
requirements-dev.txt	Replace mypy dependency with ty.
pyproject.toml	Drop [tool.mypy] config; add [tool.ty.environment] and [tool.ty.analysis] with allowed unresolved imports.
.github/workflows/app-tests.yaml	Remove mypy cache step; run ty check with GitHub output format.
.github/PULL_REQUEST_TEMPLATE.md	Update checklist to reference ty check.
.github/copilot-instructions.md	Update type-checking commands/notes to use ty.
.vscode/settings.json	Stop hiding .mypy_cache from explorer.
.gitignore	Remove mypy cache ignore entries.
src/backend/fastapi_app/rag_simple.py	Import RawResponsesStreamEvent and use isinstance for stream-event narrowing.
src/backend/fastapi_app/rag_advanced.py	Same isinstance-based stream-event narrowing change.
evals/generate_ground_truth.py	Replace # type: ignore[list-item] with # ty: ignore[invalid-argument-type].

Copilot's findings

• Files reviewed: 9/10 changed files
• Comments generated: 0