fix: fallback to lenovo fw config for lenovo-ami machines

[krish-nvidia]

Description

This PR fixes LenovoAMI firmware config lookup by falling back to Lenovo firmware metadata for the same hardware model when no LenovoAMI-specific entry exists.

This lets machine update manager find deployed Lenovo firmware bundles for Lenovo AMI BMC endpoints while preserving exact LenovoAMI configs if they are added later. Added regression coverage for fallback behavior and exact-match precedence.

Type of Change

• Add - New feature or capability
• Change - Changes in existing functionality
• Fix - Bug fixes
• Remove - Removed features or deprecated functionality
• Internal - Internal changes (refactoring, tests, docs, etc.)

Related Issues (Optional)

Breaking Changes

• This PR contains breaking changes

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• No testing required (docs, internal refactor, etc.)

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