lsp: validate range bounds in Locate to avoid panic on malformed LSP ranges

[SAY-5]

Fixes #186.

LSPClient.Locate indexed f.LineCounts[range.Line] and sliced text[start:end] without validating the range. When pylsp (jedi/parso) returns a malformed range — e.g. [13:6] — abcoder hard-crashes during parse python. Validate line indices and the resulting offsets, returning an error so the collector can skip the token instead.

[CLAassistant]

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