fix(runtime): reject duplicate in-flight task IDs (#257)

[Glastonburyk]

Summary

Addresses #257 by rejecting a second executeTask() invocation while the same task ID is already active. The duplicate call fails with typed DUPLICATE_IN_FLIGHT_TASK before duplicate tool execution or duplicate failed lifecycle events can occur. Task IDs can be reused after the previous execution settles.

Regression coverage launches concurrent duplicate IDs, checks the typed error and active task count, confirms no duplicate runtime.task.failed event, and verifies reuse after settlement.

Verification

The initial isolated run exposed two TypeScript defects in the test. Those were repaired. The exact final patch then passed the repository's complete npm run verify gate with formatting, ESLint, TypeScript, Vitest and coverage under network-disabled repository execution.

This is an independent competing submission for the still-open bounty.

Closes #257

[Glastonburyk]

Thanks for merging this. Could you confirm whether there is anything else required from me to complete the bounty settlement/payment, and what the next step is? If payout details are needed, please point me to the appropriate official/private route.