[cumminskd]

Summary

PromptSession.prompt() and prompt_async() currently treat an omitted validator argument and an explicit validator=None the same way. As a result, a validator configured on a reusable session cannot be cleared for a later prompt.

This change distinguishes those two call forms:

• Omitting validator preserves the session validator.
• Passing validator=None clears the session validator.
• Passing another validator replaces it.

The behavior is implemented for both synchronous and asynchronous prompts, with regression coverage in tests/test_cli.py and updated API documentation.

Fixes #2159

Verification

• Focused CLI tests: 37 passed
• Full test suite: 159 passed
• Ruff: passed
• Mypy: passed across 145 source files
• A pre-fix probe reproduced the original explicit-None failure
• Omitted-validator preservation and non-None replacement were also verified for sync and async prompts

Related work

PR #2161 is acknowledged as a documentation-based alternative. It documents the existing workaround and tutorial guidance without changing runtime behavior. This PR addresses the underlying runtime behavior requested in #2159.

Backwards-compatibility consideration

This intentionally changes the meaning of an explicit validator=None passed to prompt() or prompt_async(): it now clears the session validator. Code that previously relied on None leaving an existing validator active should omit the argument instead. Calls that omit validator, pass a validator, or use the existing completer and auto-suggest behavior retain their prior semantics.