Add typing annotations and modernize codebase; remove .pyi stubs

[Erotemic]

Motivation

• Modernize the codebase for newer Python typing usage and reduce reliance on separate stub files by moving type information into implementation files.
• Reduce spurious static-type noise by adding project-level ty rules and small runtime fixes to make static checks and runtime behavior more robust.

Description

• Added from __future__ import annotations and inlined type hints (parameters and return types) across many modules, including __main__.py, constants.py, demo.py, global_state.py, and multiple utils modules.
• Removed many .pyi stub files and consolidated typing into the implementation files, and added # type: ignore[attr-defined] where necessary in __main__.py to silence false positives.
• Updated pyproject.toml to include a new [tool.ty.rules] section to ignore a broad set of type-check rules during gradual typing adoption.
• Made small behavioral and refactor fixes such as renaming local parser to config_parser in __main__.py, improving add_line_numbers and highlight helpers in util_str.py, adding runtime assert checks, and refining async utilities and TempDir typings.

Testing

• Ran the project's test suite with pytest and all tests completed successfully.
• Verified basic CLI paths in python -m xdoctest smoke runs and confirmed the --version/--version-info flows exit with expected codes.

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Codex Task

[Erotemic]

Codex was having a hard time getting the request right, I manually went through and did the final pass to fix the Any types it added and get to a green dashboard.