feat: add comprehensive type checking with mypy strict mode and pyright

[mldangelo]

Summary

This PR enhances the type checking infrastructure by:

• Upgrading mypy to strict mode with additional error codes (ignore-without-code, redundant-cast, truthy-bool, truthy-iterable, unused-awaitable)
• Adding pyright as a second type checker for comprehensive coverage, running in strict mode
• Updating CI to run both type checkers in parallel via a matrix strategy for efficient execution
• Fixing source code type issues to pass both strict checkers

Why Two Type Checkers?

mypy and pyright catch different categories of issues:

Checker	Strengths
mypy	Standard Python type checker, excellent for protocol validation, widely adopted
pyright	Microsoft's type checker, faster execution, catches additional edge cases, better IDE integration

Running both provides defense-in-depth: errors caught by one may be missed by the other.

Configuration Details

mypy (strict mode + extras):

• All flags from --strict enabled
• Additional error codes for comprehensive coverage
• Override for posthog.* module to ignore missing imports

pyright (strict mode with practical exceptions):

• typeCheckingMode = "strict"
• Disabled checks for unknown types from third-party libraries lacking full stubs
• Compatible with mypy's type: ignore comments

Changes

• pyproject.toml: Enhanced mypy config, added pyright config and dependency
• .github/workflows/test.yml: Matrix strategy for parallel type checker execution
• src/promptfoo/cli.py: Fixed CompletedProcess generic type parameter
• src/promptfoo/telemetry.py: Added type: ignore for untyped posthog methods
• AGENTS.md: Updated documentation with new type checking setup
• uv.lock: Added pyright dependency

Test plan

• Both uv run mypy src/promptfoo/ and uv run pyright src/promptfoo/ pass locally
• Ruff linting and formatting checks pass
• CI runs both type checkers in parallel
• All existing tests continue to pass

🤖 Generated with Claude Code