refactor(llm): bundle file saver dependencies

[laipz8200]

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Related Issue

Refs #66

Summary

• add FileSaverDependencies so callers can pass one explicit runtime bundle to FileSaverImpl
• keep the legacy keyword constructor path while rejecting mixed or incomplete construction
• add focused file-saver constructor coverage and switch the default-path HTTP client test to the new call site

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation