fix: fix imageModelProvider apiKey selection fallback

[its-DeFine]

##Relates to https://github.com/ai16z/eliza/issues/1270

##Risks None known

What does this PR do?

Introduces an addition on packages/core/src/generate.ts that first tries to match the apiKey to it's corresponding imageModelProvider and moves to the fallback logic if that fails.

What kind of change is this?

Bug fixes (non-breaking change which fixes an issue)

Why are we doing this? Any context or related work?

Current behaviour is not intended and causing issues and confusion.

Documentation changes needed?

Testing

Manually tested

Where should a reviewer start?

packages/core/src/generate.ts

Detailed testing steps

Deploy Notes

Database changes

Discord username

[its-DeFine]

Closed to review again and remediate issues

[Titan-Node]

+1 to this pull request. Looking to add another image gen provider but waiting for this pull to fix an underlying issue.