fix(picker): warn before falling back when a provider errors

[binbandit]

Summary

M.pick() wrapped every provider call in pcall and walked the fallback chain on any failure, which conflated two very different cases:

• Capability gap — the provider doesn't implement the kind (fff has no buffers, so it falls through to snacks). This is by design, matches the documented fallback chain, and stays silent.
• Real error — the provider implements the kind but the call blew up. Previously this was swallowed and the next provider quietly took over, hiding a broken configuration behind a picker that merely looked slightly different. The manifesto's "we do not hide configuration behind magic" says this should be visible.

Now an erroring provider produces a warning with the provider name, the kind, and the error message before the dispatcher tries the next one — same pattern core/terminal.lua already uses for its snacks→native fallback. Fallback order and behavior are otherwise unchanged.

Also documents the distinction in the Fallback chain section of docs/src/content/docs/guide/pickers.md.

Validation

make validate passes. Neovim isn't available here for make smoke; manual check: with the default fff provider, :BlakPick buffers should open snacks with no warning (capability fallback), and breaking a provider (e.g. a bad fff install) should produce one warning per failed pick rather than silence.