fix: warn when [[providers.local]] discards its fields

A malformed local entry was the one skip that said nothing, on the
reasoning that local still ends up working. It does, but the merge
rebuilds it from the bare base_url, so an api_key, filter or ctx_size
set on that entry is dropped without a word. The app then comes up
looking healthy, which is the strongest possible signal that nothing is
wrong, making this the case that most needs saying, not least.

Local surviving at all depends on DEFAULTS supplying base_url, since
that is what the rebuild reads. Noted in config.py so the key is not
removed as redundant, and pinned by a test.

The not-a-table wording now follows what was written: a list gets the
single-bracket fix by name, while a scalar entry does not, since telling
someone to fix a [[...]] they never typed points at the wrong line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0176tzAW6H1i2Kz8vm2XXVGV

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