Fix: Persist total API cost after message deletion

[abdulrahimpds]

This PR fixes an issue where the "Total API Cost" would incorrectly decrease when chat messages were deleted, instead of showing the real resource usage for the session. The cost tracking now keeps costs from deleted messages.

• Backend (Task.ts): Added a _deletedApiCost property to keep track of costs from deleted messages and updated getTokenUsage to include this value.
• Logic (webviewMessageHandler.ts): Hooked into message deletion to calculate and save the cost of messages before they are removed.
• Frontend (ChatView.tsx): Updated the cost display to use the backend’s currentTaskCumulativeCost as the source of truth instead of recalculating it from the visible messages.

The "Total API Cost" now correctly shows the full cost of the session, even if messages are deleted.

[changeset-bot]

🦋 Changeset detected

Latest commit: 90a4495

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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kilo-code	Patch

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[kevinvandijk]

Thanks @abdulrahimpds! It took a while to get around to this one but nice fix. If you feel like, reach out to me on Discord (Kevin) with the email address you used to sign up for Kilo so I can grant you some credits as a token of our appreciation!