Warn when selector contains unaccompanied graph operator

[viniciusnunest]

Resolves #10388

Problem

If you run something like dbt list --select + or dbt list --select 1+1,
dbt just accepts it silently and gives you unexpected results. Usually nothing,
sometimes everything downstream. No warning, no hint that the selector is wrong.

The original report was from someone who put an accidental space in an intersection
selector: model+,+ other_model. That space caused + other_model to be parsed
as just +.

Solution

Added detection for these "unaccompanied" graph operators in
SelectionCriteria.from_single_spec. When one is found, it fires a
NoNodesForSelectionCriteria warning, the same warning dbt already uses when
a selector matches zero nodes. Felt like the right fit instead of creating a
whole new event type.

A graph operator counts as unaccompanied when:

• It has + or @ but no actual value (+, @, 1+)
• The value is purely numeric, so it looks like a depth modifier instead of
  a model name (1+1, +2, 2+3)

Anything with letters, paths, or colons in the value passes through without
warning. So +mymodel, tag:my_tag, even +1abc are all fine.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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