feat: warn about parametrization IDs that break -k selection

[royy92]

What

Add a warning for custom parametrization IDs that cannot be safely used as part of a -k identifier.

Also document guidance for choosing custom IDs that work with -k.

Why

Custom parametrization IDs can contain characters such as parentheses, commas, whitespace, or = that have special meaning in -k expressions.

This can produce valid collected test IDs that cannot be selected directly with -k, as reported in #6626.

Following the discussion in #6626, this starts with warnings for custom IDs only. Built-in generated IDs are left unchanged for separate consideration.

How

Add is_safe_identifier_part() using the existing -k expression scanner to check whether a custom ID can be safely embedded in an identifier.

Warn with PytestWarning when a custom ID that prevents direct -k selection comes from:

• pytest.param(..., id=...).
• ids=[...].
• an ids callable.
• pytest_make_parametrize_id.

Built-in generated IDs are not warned about.

Test

Add coverage for:

• safe and unsafe identifier parts.
• explicit pytest.param IDs.
• IDs provided through ids=[...].
• callable-generated IDs.
• IDs returned by pytest_make_parametrize_id.
• safe custom IDs such as and, or, not, and an empty ID.
• unselectable built-in generated IDs remaining warning-free.

The relevant test suites pass with 304 tests.

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