fix(go/ai): genkit ignores dotprompt-defined roles

[josearomeroj]

Fixed Genkit's prompt load to respect role definitions from dotprompt files instead of forcing all messages to use the "user" role.

#3711

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[hugoaguirre]

Thanks for your contribution! Overall, LGTM 👍🏽

[eric642]

@hugoaguirre @rutujaD1999 @apascal07 My actual verification did not solve this problem, and this submission introduced a new bug. The modified code has caused the shared variable to behave in such a way that when multiple threads access the same prompt simultaneously, the message remains unchanged indefinitely.

if len(nonSystemMessages) > 0 {
	promptOpts = append(promptOpts, WithMessages(nonSystemMessages...))
} 

[apascal07]

@rutujaD1999 please take a look.