feat(js/genkit): Added cancelOperation for backgroundModels

[ifielker]

Usage

This PR introduces ai.cancelOperation(), allowing you to programmatically cancel long-running operations for background models that support it.

const ai = genkit({
  plugins: [<your background model plugin>],
});

// Start a long-running operation
let { operation } = await ai.generate({ <your background model config and request> });

// ... later, if you need to cancel it ...
const canceledOperation = await ai.cancelOperation(operation);

console.log('Operation canceled:', canceledOperation);

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)

[gemini-code-assist]

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