fix: use Promise.allSettled for multi-agent partial failure handling

[cparnell-scope3]

Summary

Bug: When using AgentCollection methods (e.g., allAgents().getProducts()) across multiple agents, if any single agent threw an exception, Promise.all would reject immediately and discard all results—even from agents that succeeded.

Fix: Replace Promise.all with Promise.allSettled so that:

• All agent calls complete independently (success or failure)
• Successful agents return their results normally
• Failed agents return a TaskResult with success: false, the error message, and agent metadata

Changes:

• Add executeAllSettled private method for consistent partial failure handling
• Update all AgentCollection task methods to use executeAllSettled
• Update execute() to return PromiseSettledResult[] for custom executors
• Add unit tests covering partial failure scenarios

Test plan

• Build passes (npm run build)
• New unit tests pass for partial failure scenarios
• Verify successful agents return results when one agent fails
• Verify failed agent results contain error message and metadata
• Verify execute() returns PromiseSettledResult array

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[cparnell-scope3]

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