refactor: eliminate duplicate code across LLM provider and agent layers - #108

issues found

Consider pre-allocating `chunks` (prealloc)

Error return value is not checked (errcheck)

unnamedResult: consider giving a name to these results (gocritic)

httpNoBody: http.NoBody should be preferred to the nil request body (gocritic)

shadow: declaration of "err" shadows declaration at line 195 (govet)

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