fix: let selector arguments accept an explicit null

[vishalhabib99]

ClickBySelector and WaitForElement type their four selector arguments (text, resourceId, className, description) as str with a None default. The generated schema therefore advertises "default": null on a field whose type is "string". A client or agent framework that sends that default back explicitly gets a validation error, and the tool never runs.

Repro (offline, no device needed; FastMCP's in-memory Client on current master, 0640a5d):

async with Client(mcp) as c:
    await c.call_tool("WaitForElement", {"text": "OK", "resourceId": None}, raise_on_error=False)

1 validation error for call[wait_for_element_tool]
resourceId
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]

ClickBySelector fails the same way. Both tools already treat a missing selector as "not given" (if text: ...), so null should mean the same thing.

Change: the 8 arguments are now str | None = None. Nothing else changes.

Tests (tests/unit/test_tool_null_args.py):

• every tool parameter that defaults to None allows null in its schema (enumerates all tools, so a new one can't slip back in)
• both tools get past argument validation with explicit nulls (the device lookup is stubbed, so no device is needed)

All 3 fail on master and pass with the fix. Full suite: 33 passed. Ruff on src/: 46 → 38 findings (the 8 RUF013 implicit-Optional warnings on these arguments are gone; nothing new).

Same fix as CursorTouch/MacOS-MCP#69, found the same way (mcp-doctor's none_default_type check, then reproduced).